These specify that they are needed by the test block. This can be
combined with `:build` to ensure that this formula isn't uninstalled
by `brew test-bot` when running `test do` blocks on our CI.
While it may suffice to merge string and non-reserved tags by forming a
union of all tags of dependencies of the same name, this approach fails
to work for the reserved tags. These are now merged such that the most
restrictive tag (meaning sometimes an empty tag) is preserved.
The previous behavior caused essential dependencies to be omitted and
builds to fail in response. E.g., multiple `:fortran` dependencies with
tags `[]`, `[:recommended]`, and `[:optional]` would have been expanded
and merged to `"gcc"` with tags `[:recommended, :optional]`, causing it
to be no longer seen as a required dependency.
ClosesHomebrew/homebrew#47040.
Signed-off-by: Martin Afanasjew <martin@afanasjew.de>
