Refactor away rspec-wait

[dduugg]

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Per the suggestion of @MikeMcQuaid, this PR removes rspec-wait. The test dependency dates to at least 2016, but since there is now a built-in timeout mechanism, I converted the existing use of wait to that style instead. (I confirmed that I was able to induce a test failure with a sufficiently low timeout value.)

[MikeMcQuaid]

Great work again @dduugg!