Auto merge of #13798 - bholley:mach_bench, r=Wafflespeanut
Auto merge of #13798 - bholley:mach_bench, r=Wafflespeanut

Implement ./mach test-unit --bench

Currently there's no way to run rust benchmark tests using mach. We can support it pretty easily, which I found useful when writing performance tests.

r? @Wafflespeanut

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