tests: speed-up by splitting across the matrix

[slint]

Tests in invenio-rdm-records are currently too slow, taking ~20min to fully run. This is very disruptive for integrating PRs and hinders review/merge/release speed significantly.

We can "horizontally" scale the tests in GH actions by splitting the tests across multiple workers/runners. I see two approaches to split:

• based on directories (e.g. tests/services, tests/resources)
• "round-robin"-style as e.g. described in https://guicommits.com/parallelize-pytest-tests-github-actions/

[github-actions]

This issue was automatically marked as stale.