tests, isisd: add IS-IS SPF unit tests

tests, isisd: add IS-IS SPF unit tests

Now that the IS-IS SPF code is more modular, write some unit tests
for it.

This commit includes a new test program called "test_isis_spf" which
can load any test topology (there are 13 different ones available)
and run SPF on any desired node. In the future this same test program
and topologies will also be used to test reverse SPF and TI-LFA.

The "test_common.c" file contains helper functions used to parse the
topology descriptions from "test_topologies.c" into LSP databases
that can be used as an input to the SPF code.

This commit also introduces the F_ISIS_UNIT_TEST flag which is used
to prevent the IS-IS code from scheduling any event when running
under the context of an unit test.

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