PYTHON-2764 Fix unified test coerce_result on unack writes (#652)

(cherry picked from commit 3ef0117)

Author: ShaneHarvey

test/unified_format.py

@@ -614,6 +614,8 @@ def match_event(self, event_type, expectation, actual):
 
 def coerce_result(opname, result):
     """Convert a pymongo result into the spec's result format."""
+    if hasattr(result, 'acknowledged') and not result.acknowledged:
+        return {'acknowledged': False}
     if opname == 'bulkWrite':
         return parse_bulk_write_result(result)
     if opname == 'insertOne':