Add `--toolchain` option for `test` command #1420

Job	Run time
build (--std-tests)	44s
build (--test-successful-rustc --nb-parts 2 --current-part 1)	2m 24s
build (--test-successful-rustc --nb-parts 2 --current-part 0)	46s
build (--mini-tests)	47s
build (--test-libcore)	49s
success_gcc12	4s
	5m 34s

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