tests: add topotest for IS-IS SR

tests: add topotest for IS-IS SR

This commit introduces a comprehensive test for IS-IS Segment
Routing. It features the following network topology:

                         +---------+
                         |         |
                         |   RT1   |
                         | 1.1.1.1 |
                         |         |
                         +---------+
                              |eth-sw1
                              |
                              |
                              |
         +---------+          |          +---------+
         |         |          |          |         |
         |   RT2   |eth-sw1   |   eth-sw1|   RT3   |
         | 2.2.2.2 +----------+----------+ 3.3.3.3 |
         |         |     10.0.1.0/24     |         |
         +---------+                     +---------+
    eth-rt4-1|  |eth-rt4-2          eth-rt5-1|  |eth-rt5-2
             |  |                            |  |
  10.0.2.0/24|  |10.0.3.0/24      10.0.4.0/24|  |10.0.5.0/24
             |  |                            |  |
    eth-rt2-1|  |eth-rt2-2          eth-rt3-1|  |eth-rt3-2
         +---------+                     +---------+
         |         |                     |         |
         |   RT4   |     10.0.6.0/24     |   RT5   |
         | 4.4.4.4 +---------------------+ 5.5.5.5 |
         |         |eth-rt5       eth-rt4|         |
         +---------+                     +---------+
       eth-rt6|                                |eth-rt6
              |                                |
   10.0.7.0/24|                                |10.0.8.0/24
              |          +---------+           |
              |          |         |           |
              |          |   RT6   |           |
              +----------+ 6.6.6.6 +-----------+
                  eth-rt4|         |eth-rt5
                         +---------+

Each router has both an IPv4 and an IPv6 loopback address, each of
which has a corresponding Prefix-SID configured.

Basic SR functionality is tested. First, the topotest ensures that
all expected Prefix-SIDs and Adj-SIDs are flooded and installed
correctly in all routers of the network. Later, network failures
are simulated and configuration changes are performed in several
different routers. The topotest then checks if IS-IS has converged
as expected in the network according to the network failures and
configuration changes that happened.

The topotest comprises 11 different steps, each with four individual
tests (for a total of 44 tests). A summary of what each step does
is provided below:

---

STEP 1:
-Initial network convergence

---

Step 2:
Action(s):
-Disable IS-IS on the eth-rt5 interface on rt4

Expected changes:
-rt4 should uninstall the Adj-SIDs pointing to rt5
-rt5 should uninstall the Adj-SIDs pointing to rt4
-rt2 should reinstall rt5's Prefix-SIDs (2 nexthops deleted)
-rt3 should reinstall rt4's Prefix-SIDs (2 nexthops deleted)
-rt4 should reinstall rt3's Prefix-SIDs (1 nexthop deleted)
-rt4 should reinstall rt5's Prefix-SIDs (1 nexthop changed)
-rt5 should reinstall rt2's Prefix-SIDs (1 nexthop deleted)
-rt5 should reinstall rt4's Prefix-SIDs (1 nexthop changed)

---

Step 3:
Action(s):
-Shut down the eth-rt4 interface on rt6
-Shut down the eth-rt5 interface on rt6

Expected changes:
-All routers should uninstall rt6's Prefix-SIDs
-rt4 and rt5 should uninstall the Adj-SIDs pointing to rt6
-rt4 should reconverge rt5's Prefix-SIDs through rt2 using ECMP
-rt5 should reconverge rt4's Prefix-SIDs through rt3 using ECMP
-rt6 should uninstall all its IS-IS routes, Prefix-SIDs and Adj-SIDs

---

Step 4:
Action(s):
-Bring up the eth-rt4 interface on rt6
-Bring up the eth-rt5 interface on rt6
-Change rt6's SRGB

Expected changes:
-All routers should install rt6's Prefix-SIDs
-rt4 and rt5 should install Adj-SIDs for rt6
-rt4 should reconverge rt5's Prefix-SIDs through rt6 using the new SRGB
-rt5 should reconverge rt4's Prefix-SIDs through rt6 using the new SRGB
-rt6 should reinstall all IS-IS routes and Prefix-SIDs from the network,
and Adj-SIDs for rt4 and rt5

---

Step 5:
Action(s):
-Disable SR on rt6

Expected changes:
-All routers should uninstall rt6's Prefix-SIDs
-rt4 should uninstall rt5's Prefix-SIDs since the nexthop router hasn't
SR enabled anymore
-rt5 should uninstall rt4's Prefix-SIDs since the nexthop router hasn't
SR enabled anymore
-rt6 should uninstall all Prefix-SIDs from the network, and the Adj-SIDs
for rt4 and rt5

---

Step 6:
Action(s):
-Enable SR on rt6

Expected changes:
-All routers should install rt6's Prefix-SIDs
-rt4 should install rt5's Prefix-SIDs through rt6
-rt5 should install rt4's Prefix-SIDs through rt6
-rt6 should install all Prefix-SIDs from the network, and Adj-SIDs for
rt4 and rt5

---

Step 7:
Action(s):
-Delete rt1's Prefix-SIDs

Expected changes:
-All routers should uninstall rt1's Prefix-SIDs

---

Step 8:
Action(s):
-Re-add rt1's Prefix-SIDs

Expected changes:
-All routers should install rt1's Prefix-SIDs

---

Step 9:
Action(s):
-Change rt1's Prefix-SIDs to use the no-php option
-Change rt6's Prefix-SIDs to stop using the explicit-null option

Expected changes:
-rt2 and rt3 should reinstall rt1's Prefix-SIDs accordingly
-rt4 and rt5 should reinstall rt6's Prefix-SIDs accordingly

---

Step 10:
Action(s):
-Remove the IPv4 address from rt4's eth-rt2-1 interface

Expected changes:
-rt2 should uninstall the IPv4 Adj-SIDs attached to the eth-rt4-1
interface
-rt2 should reinstall all IPv4 Prefix-SIDs whose nexthop router is rt4
(ECMP shouldn't be used anymore)
-rt4 should reinstall all IPv4 Prefix-SIDs whose nexthop router is rt2
(ECMP shouldn't be used anymore)

---

Step 11:
Action(s):
-Restore the original network setup

Expected changes:
-All routes, Prefix-SIDs and Adj-SIDs should be the same as they were
after the initial network convergence (step 1)

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>