tests: made a couple of prechecks consistent with others

Also removed a TODO suggesting caching the precheck results. Tests
showed this would save about 0.1 sec on the total test run time on a
relatively modern system, an unnoticeable gain at the cost of longer and
more complicated code. There would also be a danger that a cached test
result would be inappropriately returned, such as when other test
dependencies (like environment variables) are different or when the
precheck causes side effects (like filesystem changes).

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