Simplify running tests #1984

Job	Run time
build (--mini-tests)	52s
build (--test-libcore)	40s
build (--test-successful-rustc --nb-parts 2 --current-part 1)	53s
build (--test-successful-rustc --nb-parts 2 --current-part 0)	41s
build (--std-tests)	50s
success_gcc12	2s
	3m 58s

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