Fix flaky tests of explain command

The server response now includes operationTime and $clusterTime, which
usually won't change in a test scenario but occasionally do.  The
tests now remove those fields from explain responses before they are
compared.

Author: jyemin

driver-core/src/test/functional/com/mongodb/operation/QueryOperationHelper.groovy

@@ -25,6 +25,8 @@ class QueryOperationHelper {
         document.remove('executionStats')
         document.remove('serverInfo')
         document.remove('executionTimeMillis')
+        document.remove('operationTime')
+        document.remove('$clusterTime')
         document
     }