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
- Move HOMEBREW_TAP_DIRECTORY to startup/config.rb because this file
holds more of the directory constants
- Rename `Commands.cmd_directories` to `Commands.tap_cmd_directories`
to better express that the commands come from taps
This file has the directory constants while the other one has regexes.
Just better organization.
