Enable xfail_strict pytest config by default

[giang-nghg]

Fixes #7065

[astropy-bot]

Hi there @giang-nghg 👋 - thanks for the pull request! I'm just a friendly 🤖 that checks for issues related to the changelog and making sure that this pull request is milestoned and labeled correctly. This is mainly intended for the maintainers, so if you are not a maintainer you can ignore this, and a maintainer will let you know if any action is required on your part 😃.

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[pllim]

Thank you for the PR!

These are reported as XPASS (in astropy/coordinates/tests/test_sky_coord_velocities.py) and will need to be addressed by coordinates experts:

=================================== FAILURES ===================================
__________________________ test_transforms_diff[sc1] ___________________________
[XPASS(strict)] 
__________________________ test_transforms_diff[sc3] ___________________________
[XPASS(strict)] 

[pllim]

I have incorporated your commit into #7267 . FYI. Once (if) that is merged, this will be closed. However, you will still be credited because I did a cherry pick. Thank you again for your contribution!