PYTHON-2635 Unpin sessions after all abortTransaction attempts (#609)

Add unified test runner for transactions.

(cherry picked from commit 61c6876)

Author: ShaneHarvey

pymongo/client_session.py

@@ -660,6 +660,7 @@ def abort_transaction(self):
             pass
         finally:
             self._transaction.state = _TxnState.ABORTED
+            self._unpin_mongos()
 
     def _finish_transaction_with_retry(self, command_name):
         """Run commit or abort with one retry after any retryable error.

test/test_transactions.py

@@ -39,10 +39,13 @@
                         wait_until, OvertCommandListener,
                         TestCreator)
 from test.utils_spec_runner import SpecRunner
+from test.unified_format import generate_test_classes
 
 # Location of JSON test specifications.
-_TEST_PATH = os.path.join(
-    os.path.dirname(os.path.realpath(__file__)), 'transactions')
+TEST_PATH = os.path.join(
+    os.path.dirname(os.path.realpath(__file__)), 'transactions', 'legacy')
+UNIFIED_TEST_PATH = os.path.join(
+    os.path.dirname(os.path.realpath(__file__)), 'transactions', 'unified')
 
 _TXN_TESTS_DEBUG = os.environ.get('TRANSACTION_TESTS_DEBUG')
 
@@ -466,13 +469,17 @@ def run_scenario(self):
     return run_scenario
 
 
-test_creator = TestCreator(create_test, TestTransactions, _TEST_PATH)
+test_creator = TestCreator(create_test, TestTransactions, TEST_PATH)
 test_creator.create_tests()
 
 
 TestCreator(create_test, TestTransactionsConvenientAPI,
             TestTransactionsConvenientAPI.TEST_PATH).create_tests()
 
 
+# Generate unified tests.
+globals().update(generate_test_classes(UNIFIED_TEST_PATH, module=__name__))
+
+
 if __name__ == "__main__":
     unittest.main()