Mark pipeline tests to skip them easily
# What does this PR do?

This PR re-introduces the `is_pipeline_test` marker to easily flag pipeline tests. The problem is that since #21516 pipeline tests aren't isolated in the pipelines folder anymore (it looks like it but there is an inheritance of the pipeline tester in all model test classes). Thus the `is_pipeline_test` will allow us to flag those tests.

By setting the RUN_PIPELINE_TESTS environment variable to False, we can skip all pipeline tests. Contrarily to other similar env variables, this one defaults to True. This is because it's very annoying to have to remember to add a `RUN_PIPELINE_TESTS=yes` before the pytest command when trying to debug things locally. I'll actually propose to switch the defaults of all other env variables in a follow-up PR. The main thing is to set it to False when running test jobs unrelated to pipelines (which this PR does).


_The documentation is not available anymore as the PR was closed or merged._