Tests: Refactor tearDown show command logging, add lifecycle markers.

This change adds a consistent interface for adding test-specific show commands to
log.txt.

It also adds log markers for the execution of setUp[Class], tearDown[Class]
in the logs.

Change-Id: I7d42e396e594a59e866a7d55dac0af25548e657a
Signed-off-by: Paul Vinciguerra <pvinci@vinciconsulting.com>
diff --git a/test/test_ip6_vrf_multi_instance.py b/test/test_ip6_vrf_multi_instance.py
index 45c192d..c4b057b 100644
--- a/test/test_ip6_vrf_multi_instance.py
+++ b/test/test_ip6_vrf_multi_instance.py
@@ -176,9 +176,10 @@
         Show various debug prints after each test.
         """
         super(TestIP6VrfMultiInst, self).tearDown()
-        if not self.vpp_dead:
-            self.logger.info(self.vapi.ppcli("show ip6 fib"))
-            self.logger.info(self.vapi.ppcli("show ip6 neighbors"))
+
+    def show_commands_at_teardown(self):
+        self.logger.info(self.vapi.ppcli("show ip6 fib"))
+        self.logger.info(self.vapi.ppcli("show ip6 neighbors"))
 
     def create_vrf_and_assign_interfaces(self, count, start=1):
         """