Fix a TF test name (LayoutLMModelTest)
# What does this PR do?

Change `LayoutLMModelTest` to `TFLayoutLMModelTest` in `test_modeling_tf_layoutlm.py`.


_The documentation is not available anymore as the PR was closed or merged._<|||||>merge now (think it's not necessary to wait Matt's approval for this tiny and obvious fix :-) )