chore: set up concurrency limit for tests

[Panaetius]

this PR sets concurrency for the main tests, so old runs get properly cancelled if there's a new push.

I also changes it from group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }} to group: ${{ github.workflow }}-${{ github.ref || github.run_id }}. The former is I think just copied from a stack overflow answer, but using github.ref makes more sense, as github.ref is already refs/pull/<pr_number>/merge, so unique per PR. run_id is used as a safety fallback for things that aren't PRs where we don't want concurrency.

[coveralls]

Pull Request Test Coverage Report for Build 12121522276

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.006%) to 85.862%

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Files with Coverage Reduction	New Missed Lines	%
components/renku_data_services/crc/blueprints.py	1	93.06%

Totals
Change from base Build 12121462595:	0.006%
Covered Lines:	14496
Relevant Lines:	16883

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💛 - Coveralls