Tests' handling of timeouts, temp dirs flaky on linux #410

I'm getting inconsistent results running the test suite on Arch Linux since 4f57594, and subsequent commits don't seem to fix it.
These seem to be localised to Ghcid basic and Cd basic, with the former sometimes timing out after 30s (despite me trying to bump the timeouts in all places I found them) and both sometimes complaining the temporary files they create have vanished from under them.

Moreover, API test has started complaining that eg

Couldn't run test because test source is missing, test/bar

between two runs of No files?

I've also seen (much more rarely)

    Ghcid .ghci
      Exception: test/foo/./Paths.hs: copyFile:atomicCopyFileContents:withReplacementFile:copyFileToHandle:openFdAt: does not exist (No such file or directory)

Unclear what's going on, my usual strategies for trying to isolate the problem are failing.