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_l2bd_arp_term.py b/test/test_l2bd_arp_term.py
index 9a14d1d..a3feb99 100644
--- a/test/test_l2bd_arp_term.py
+++ b/test/test_l2bd_arp_term.py
@@ -67,9 +67,10 @@
         Show various debug prints after each test.
         """
         super(TestL2bdArpTerm, self).tearDown()
-        if not self.vpp_dead:
-            self.logger.info(self.vapi.ppcli("show l2fib verbose"))
-            self.logger.info(self.vapi.ppcli("show bridge-domain 1 detail"))
+
+    def show_commands_at_teardown(self):
+        self.logger.info(self.vapi.ppcli("show l2fib verbose"))
+        self.logger.info(self.vapi.ppcli("show bridge-domain 1 detail"))
 
     def add_del_arp_term_hosts(self, entries, bd_id=1, is_add=1, is_ipv6=0):
         for e in entries: