CI: Remove the pytest-xdist plugin from the "Tests" and "Dev Tests" workflows

[seisman]

The macOS CI jobs recently started failing, as originally reported in https://github.com/GenericMappingTools/pygmt/pull/3601#issuecomment-2466212811. After investigating, I identified commit f91475d6611999e84c1e65a702db1d65b3e254a8 as the first commit causing the issue. However, reverting this commit alone does not fully resolve the failures, as subsequent commits related to testing the _to_numpy function also contribute to the problem.

The exact reason why commit f91475d6611999e84c1e65a702db1d65b3e254a8 triggers these failures (on macOS only) remains unclear. However, removing the pytest-xdist plugin, which runs tests across multiple processes, resolves the failures, allowing the CI jobs to pass successfully again.

The downside of this fix is that removing pytest-xdist increases the CI runtime, as tests now run sequentially instead of in parallel. While this trade-off impacts efficiency, it ensures stability for the macOS CI jobs.

[github-actions[bot]]

Summary of changed images

This is an auto-generated report of images that have changed on the DVC remote

Status	Path
added	pygmt/tests/baseline/test_figure_shift_origin.png
deleted	pygmt/tests/baseline/test_shift_origin.png

Image diff(s)

### Added images * test_figure_shift_origin.png  ### Modified images | Path | Old | New | |---|---|---|

Report last updated at commit 2666531c8ebf1176b2d4b0b288ec3e7c6c928004