[INFRA] Run anat non regression tests with GitHub actions

[NicolasGensollen]

Continuation of #971 but targeting Anat pipelines.

This PR proposes to:

• re-organize anat non regression tests in multiple files in an anat test module (test_t1_linear.py, test_t1_volume.py, and test_t1_freesurfer.py)
• refactor some of the tests to make code less verbose
• add a GitHub action workflow which runs the non regression tests for anat pipelines

For now the workflow is supposed to run on PRs to check that everything works as expected. Will make it a cron job once we have everything set up.