implementing tflxmertmodel integration test
# What does this PR do?
this PR implement an integration test for TFLxmertmodel as requested in #9954  

@LysandreJik 
This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md) are likely to be ignored.<|||||>@LysandreJik <|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md) are likely to be ignored.<|||||>Hi @LysandreJik  what about this one <|||||>Sure! It seems there is no branch linked to this PR, however. The changes are still visible here https://github.com/huggingface/transformers/pull/10052/files but I am unable to reopen.