[WIP] add `has_attentions` as done in PyTorch side
# What does this PR do?

My original intention is to clean up `TFModelTesterMixin.test_pt_tf_model_equivalence`. This is done in this first commit.
Would like to hear from @sgugger to see if it is better to apply the same change found in #15909 here.

 

_The documentation is not available anymore as the PR was closed or merged._<|||||>So I will merge as it is, and work in a follow up PR to use `has_attentions` to other (TF) test methods like `test_model_outputs_equivalence` etc.