Fix flaky `test_for_warning_if_padding_and_no_attention_mask`
# What does this PR do?

This tests #24510. It fails a few times (on my PRs, and once in someone's PR), and now it fails on the latest daily CI.

This test is flaky because it test the functionality of `warn_if_padding_and_no_attention_mask` which uses `logger.warning_once(warn_string)` - this uses `@functools.lru_cache(None)`.

If there is any test that triggers this warning before `test_for_warning_if_padding_and_no_attention_mask`, the cache is produced, and we won't get the expected warning in `test_for_warning_if_padding_and_no_attention_mask` then it fails.
_The documentation is not available anymore as the PR was closed or merged._