11830 make process tests more reliable

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Scope and purpose

Fixes #11830

this splits out the file_actions calculation to its own function so tricky edge cases can be tested independently, and it fixes some getrlimit bugginess on macOS

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please review

[reaperhulk]

I don't understand the twisted specific bits here, but I looked at the docs for posix_spawn and the test cases appear correct

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OK, I'm going to accept that review from Paul, because the correctness of the posix_spawn bits is the part that needs review, the "twisted-specific" bits are not interesting :)