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_span.py b/test/test_span.py
index b0ea1e5..78a4f76 100644
--- a/test/test_span.py
+++ b/test/test_span.py
@@ -61,8 +61,9 @@
 
     def tearDown(self):
         super(TestSpan, self).tearDown()
-        if not self.vpp_dead:
-            self.logger.info(self.vapi.ppcli("show interface span"))
+
+    def show_commands_at_teardown(self):
+        self.logger.info(self.vapi.ppcli("show interface span"))
 
     def xconnect(self, a, b, is_add=1):
         self.vapi.sw_interface_set_l2_xconnect(a, b, enable=is_add)