diagnostic_spec: Needs macOS
diagnostic_spec: Needs macOS

There's not much point, IMO, in testing that the return value of
check_homebrew_prefix is nil. So just skip the whole test.

Signed-off-by: Bob W. Hogg <c772a964fd55352a3510e5d535dd9ccc9ac30168@linux.com>

hanxue/linuxbrew,Linuxbrew/brew,Linuxbrew/brew,maxim-belkin/brew,maxim-belkin/brew,Linuxbrew/brew,rwhogg/brew,rwhogg/brew,hanxue/linuxbrew,hanxue/linuxbrew,rwhogg/brew,maxim-belkin/brew,Linuxbrew/brew