Add a test printHistory function

I frequently add something like this ad-hoc when debugging a test.
Make it a convenient function that's easy to add when needed, and
also run it at the completion of every test so a developer can
easily survey the logs to see what happened.

Change-Id: I3d3810f51245d92855f086b875edfd52bdd86983

tests/base.py

@@ -1369,6 +1369,7 @@ class ZuulTestCase(BaseTestCase):
         threads = threading.enumerate()
         if len(threads) > 1:
             self.log.error("More than one thread is running: %s" % threads)
+        self.printHistory()
 
     def init_repo(self, project):
         parts = project.split('/')
@@ -1670,6 +1671,17 @@ class ZuulTestCase(BaseTestCase):
                 self.log.error("No completed builds")
             raise
 
+    def printHistory(self):
+        """Log the build history.
+
+        This can be useful during tests to summarize what jobs have
+        completed.
+
+        """
+        self.log.debug("Build history:")
+        for build in self.history:
+            self.log.debug(build)
+
     def getPipeline(self, name):
         return self.sched.abide.tenants.values()[0].layout.pipelines.get(name)