tests: Ignore SAVEPOINT queries in queries_captured().

We use the queries_captured() context manager in our tests
to capture queries that happen during certain actions.  Usually
we use the list of queries to validate that we're not doing
too many database calls.

For most tests the database calls are fairly deterministic,
but SAVEPOINT-related things seem to be more random, and the
number of savepoints is usually not relevant to what the test
is trying to prevent, which is more serious problems like
O(N) database fetches.

zerver/lib/test_helpers.py

@@ -103,8 +103,8 @@ def simulated_empty_cache():
     cache.cache_get_many = old_get_many
 
 @contextmanager
-def queries_captured():
-    # type: () -> Generator[List[Dict[str, Union[str, binary_type]]], None, None]
+def queries_captured(include_savepoints=False):
+    # type: (Optional[bool]) -> Generator[List[Dict[str, Union[str, binary_type]]], None, None]
     '''
     Allow a user to capture just the queries executed during
     the with statement.
@@ -120,10 +120,11 @@ def queries_captured():
         finally:
             stop = time.time()
             duration = stop - start
-            queries.append({
-                'sql': self.mogrify(sql, params).decode('utf-8'),
-                'time': "%.3f" % duration,
-            })
+            if include_savepoints or ('SAVEPOINT' not in sql):
+                queries.append({
+                    'sql': self.mogrify(sql, params).decode('utf-8'),
+                    'time': "%.3f" % duration,
+                })
 
     old_execute = TimeTrackingCursor.execute
     old_executemany = TimeTrackingCursor.executemany