Automatically mark all WorkerState tests

[crusaderky]

Automatically mark all tests that use the ws or the ws_with_running_task fixtures.
This allows running all and only such tests with pytest -m workerstate.

This in turn will allow, in the future,

• to measure and progressively increase test coverage of WorkerState._transition_* using deterministic WorkerState tests only
• to give developers a quick way to retest after changes to the transitions machinery.

[github-actions]

Unit Test Results

See test report for an extended history of previous test failures. This is useful for diagnosing flaky tests.

       15 files  ±0         15 suites  ±0   6h 34m 14s ⏱️ + 13m 52s
  2 950 tests ±0    2 863 ✔️  - 1       85 💤  - 1  2 ❌ +2 
21 872 runs  ±0  20 852 ✔️ +2  1 018 💤  - 4  2 ❌ +2 

For more details on these failures, see this check.

Results for commit 97332b4. ± Comparison against base commit 6765e6e.

[hendrikmakait]

LGTM, great idea! I could foresee this flag getting blurry at some point in the future, but that's a problem to tackle at that point in time.