While we're here, also add `brew tests --no-parallel` which I relied
on during testing.
Pretty much anywhere we rely on a stubbed formula on disk to work: we
need to disable the API.
The way we handle boolean environment variables is a bit unfortunate.
For example, setting `HOMEBREW_EVAL_ALL=false` actually enables
`HOMEBREW_EVAL_ALL`.
Let's fix this by deprecating setting boolean environment variables to
common false-y values (`false`, `0`, `nil`, `no`, and `off`) so that we
can later ignore these false-y values when reading boolean environment
variables.
We will now build on Ventura (macOS 13) to Sequoia (macOS 15).
Runners for macOS 15-x86_64 will not be allocated by default due to a
lack of appropriate hardware.
This allows us to keep all information about timeout values here in
`brew` instead of both here and in Homebrew/core.
Will be needed after Homebrew/homebrew-core#171457.
