The existing watchlist test in `dev-cmd/livecheck_spec.rb` will only
pass if the testing environment doesn't contain a livecheck watchlist
file. When a watchlist file is present, it ends up being treated as
empty (formulae and casks aren't available in tests) and produces an
`Invalid usage: No formulae or casks to check` error instead. We don't
have to worry about a watchlist file on CI but it's a potential issue
when running `brew test` locally.
This provides a bogus `HOMEBREW_LIVECHECK_WATCHLIST` value to the
`#brew` call, to ensure that any watchlist file in the testing
environment is not used for this test.
Co-authored-by: Mike McQuaid <mike@mikemcquaid.com>
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