ovs-macros: An option to suspend test execution on error

ovs-macros: An option to suspend test execution on error

Origins for this patch are captured at
https://mail.openvswitch.org/pipermail/ovs-discuss/2019-June/048923.html.

Summarizing here, when a test fails, it would be good to pause test execution
and let the developer poke around the system to see current status of system.

As part of this patch, made a small tweaks to ovs-macros.at, so that when test
suite fails, ovs_on_exit() function will be called. And in this function, a check
is made to see if an environment variable to OVS_PAUSE_TEST is set. If it is
set, then test suite is paused and will continue to wait for user input
Ctrl-D. Meanwhile user can poke around the system to see why test case has
failed. Once done with investigation, user can press ctrl-d to cleanup the
test suite.

For example, to re-run test case 139:

export OVS_PAUSE_TEST=1
cd tests/system-userspace-testsuite.dir/139
sudo -E ./run

When error occurs, above command would display something like this:
=====================================================
Set environment variable to use various ovs utilities
export OVS_RUNDIR=/opt/vdasari/Developer/ovs/_build-gcc/tests/system-userspace-testsuite.dir/139
Press ENTER to continue:

=====================================================
And from another window, one can execute ovs-xxx commands like:
export OVS_RUNDIR=/opt/vdasari/Developer/ovs/_build-gcc/tests/system-userspace-testsuite.dir/139
$ ovs-ofctl dump-ports br0
.
.

To be able to pause while performing `make check`, one can do:
$ OVS_PAUSE_TEST=1 make check TESTSUITEFLAGS='-v'

Acked-by: Aaron Conole <aconole@redhat.com>
Signed-off-by: Vasu Dasari <vdasari@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>