Add testing guidelines, update copilot instructions for tests (#1507) #2651

Job	Run time
build (windows-latest, Release)	16m 12s
build (ubuntu-latest, Debug)	10m 24s
build (macos-latest, Release)	11m 3s
build (macos-latest, Debug)	9m 46s
build (windows-latest, Debug)	14m 31s
build (ubuntu-latest, Release)	10m 25s
publish-coverage / publish-coverage	37s
	1h 12m 58s

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