PYTHON-2915 Skip large txn test on slow Windows hosts (#788)

Author: ShaneHarvey

test/test_transactions.py

@@ -289,6 +289,8 @@ def gridfs_open_upload_stream(*args, **kwargs):
     # Require 4.2+ for large (16MB+) transactions.
     @client_context.require_version_min(4, 2)
     @client_context.require_transactions
+    @unittest.skipIf(sys.platform == 'win32',
+                     'Our Windows machines are too slow to pass this test')
     def test_transaction_starts_with_batched_write(self):
         if 'PyPy' in sys.version and client_context.tls:
             self.skipTest('PYTHON-2937 PyPy is so slow sending large '