[#2933] Run select tests sequentially #8367

Job	Run time
Build and check documentation	1m 35s
End-to-end tests, chromium	4m 5s
End-to-end tests, firefox	4m 15s
Check that documentation for configuration steps is up-to-date	0s
Run the Django test suite (main)	19m 55s
End-to-end tests, webkit	4m 27s
Run the Django test suite (elastic)	5m 28s
End-to-end tests, msedge	4m 34s
Run the Django test suite (openklant)	2m 30s
Run the Django test suite (sequential)	3m 50s
Upload coverage information to codecov	14s
Build (and push) Docker image	1m 45s
	52m 38s

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