[Sharding 4] Base testing support for sharding #5166

Job	Run time
Release builds and tests	8m 9s
Basic Checks	19m 50s
Additional Builds and Concurrency Tests	14m 15s
Measure coverage	26m 40s
miri	3m 0s
sanitize (address)	18m 23s
Slow tests	11m 18s
sanitize (address, multi-threading)	18m 47s
sanitize (leak)	7m 52s
sanitize (leak, multi-threading)	8m 13s
	2h 16m 27s

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