Ensure clean starting state in test

Since it merged, test_make_request_clears_txn_id_after_calling_app would
always fail on the first attempt in the gate, but we wouldn't notice
because it would succeed on the retry. Trouble is that some tests (like
test_catch_errors.py) would pollute the main greenthread's threadlocals.

Change-Id: Ibdafc4a321e41d6fd225c486da831b1f0fead882

test/unit/common/test_internal_client.py

@@ -458,6 +458,9 @@ class TestInternalClient(unittest.TestCase):
 
         fake_logger = debug_logger()
         logger = utils.LogAdapter(fake_logger, 'test-server')
+        # Make sure there's no transaction ID set -- other tests may have
+        # polluted the logger
+        logger.txn_id = None
         logger.debug('Before request')
         client = InternalClient(self, logger)
         client.make_request('GET', '/', {}, (200,))