Fix missing output_attentions in PT/Flax equivalence test
# What does this PR do?

In a previous PR #15841, `output_attentions` was not set (I accidentally removed the whole block containing it).
This PR sets  `output_attentions` to make the test more thorough.

The test still runs successfully with `1e-5` on both CPU/GPU. However, see the 2nd points in the remarks below.

It also adds `has_attentions` attribute to `FlaxModelTesterMixin` (as done in PyTorch's `ModelTesterMixin`).

# Remarks:
- In a follow up PR, we might use `has_attentions` in some existing methods (to make sure the attentions are only tested if `has_attentions` is `True`), see #15909
- There are 4 Flax model testers overwrite the Flax common `test_equivalence_pt_to_flax` and `test_equivalence_flax_to_pt`.
  - I will update them in a next PR.
  - These include `FlaxGPTJ` and `FlaxXGLM`, which will fail with `1e-5`. I need to debug them.
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_16271). All of your documentation changes will be reflected on that endpoint.<|||||>Think this (quite small) PR is ready. Nothing particular but adding the missing `config.output_attentions = self.has_attentions`.
The `super()` thing was discussed in #16280.

Will merge it today.