If you have `HOMEBREW_TEMP` set to a longer path (e.g. the user's
private temporary directory rather than `/private/tmp`), Sequoia will
error out on some tests when forking because the path is too long
(> 104 bytes).
To improve this situation let's:
- use a shorter path for `HOMEBREW_TEMP` by appending `/brew-tests` to
the existing `HOMEBREW_TEMP` used outside of tests rather than
creating nested temporary directories
- use the minimal amount of randomness to avoid collisions while
allowing parallel tests
- use `squeeze` to remove any double slashes in the temporary path
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