conftest: put transaction manager in its own fixture

tests/conftest.py

@@ -702,7 +702,20 @@ def xmlrpc(self, path, method, *args):
 
 
 @pytest.fixture
-def webtest(app_config_dbsession_from_env, remote_addr):
+def tm():
+    # Create a new transaction manager for dependant test cases
+    tm = transaction.TransactionManager(explicit=True)
+    tm.begin()
+    tm.doom()
+
+    yield tm
+
+    # Abort the transaction, leaving database in previous state
+    tm.abort()
+
+
+@pytest.fixture
+def webtest(app_config_dbsession_from_env, remote_addr, tm):
     """
     This fixture yields a test app with an alternative Pyramid configuration,
     injecting the database session and transaction manager into the app.
@@ -718,11 +731,6 @@ def webtest(app_config_dbsession_from_env, remote_addr):
 
     app = app_config_dbsession_from_env.make_wsgi_app()
 
-    # Create a new transaction manager for dependant test cases
-    tm = transaction.TransactionManager(explicit=True)
-    tm.begin()
-    tm.doom()
-
     with get_db_session_for_app_config(app_config_dbsession_from_env) as _db_session:
         # Register the app with the external test environment, telling
         # request.db to use this db_session and use the Transaction manager.
@@ -737,9 +745,6 @@ def webtest(app_config_dbsession_from_env, remote_addr):
         )
         yield testapp
 
-    # Abort the transaction, leaving database in previous state
-    tm.abort()
-
 
 class _MockRedis:
     """