Use pytest-xdist with one group and two workers

[stefanw]

Tests that need elasticsearch or playwright are grouped into one worker to run sequentially.

[krmax44]

Great! I tried this approach before too, but seemed to have missed some tests for the sequential group. Seems to work reliably now, also with more than two workers.

Minor things:

• I removed the -n option from pyproject.toml and set it explicitly in CI (to -n auto), as always running parallelized can cause issues with tools like the pytest debuger within VS Code.
• I also removed the warning filters. We're currently getting only four unique warnings, at least one of which is useful.
• I added pytest-cov, which properly reports coverage with parallelized runs (before, CI reported only 16 %).