7eb29367

By: Tom Sydney Kerckhove <syd@cs-syd.eu>

Add --mutation-coverage-jobs to cap coverage parallelism

Test suites with expensive per-test setup (e.g. tmp-postgres) can overwhelm
the system when getNumCapabilities children try to start at once. Expose
a knob to limit it.

mutationCheck.nix gets a coverageJobs argument that, when set, passes the
flag to both the first-suite and extra-suite coverage invocations.

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