run require tests in parallel
run require tests in parallel

the require tests are trivially parallelizeable, so we should run them
in parallel for a faster build.

On my machine, before this commit:

Finished in 108.915734s, 6.3444 runs/s, 29.2520 assertions/s.

After:

Finished in 70.861645s, 10.6828 runs/s, 44.9609 assertions/s.

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