Better test name and enable pipeline test for `pix2struct`
# What does this PR do?

In #24364, `pix2struct` test file didn't get `pipeline_model_mapping` being updated, due to the `heuristic` of finding a test class not working well (try to find the shortest test class name).

Let's try to give the test class a better/short/clear name, (despite we don't really have a base model), `Pix2StructModelTest` instead of `Pix2StructTextImageModelTest`.

This enables the script `add_pipeline_model_mapping_to_test.py` works for `pix2struct`, and then we get the pipeline test being run.
_The documentation is not available anymore as the PR was closed or merged._