tests: Clear txn id on init for all debug loggers

Since we fake out all the greenthread stuff to run in the main thread,
we can (sometimes?) find that a transaction ID has already been set,
leading to failures in test_bad_request_app_logging like

    AssertionError: b'X-Trans-Id: test-trans-id' not found
    in b'X-Trans-Id: tx...'

By resetting the logger's txn_id, we're assured that our mock will be
run and the expected transaction ID will be used.

Change-Id: I465eed5372a2a5e591f80a09676f4b7f091cd444

test/debug_logger.py

@@ -249,6 +249,9 @@ class DebugLogger(FakeLogger):
 
 
 class DebugLogAdapter(utils.LogAdapter):
+    def __init__(self, *args, **kwargs):
+        super(DebugLogAdapter, self).__init__(*args, **kwargs)
+        self.txn_id = None
 
     def __getattribute__(self, name):
         try: