dev-cmd/livecheck: avoid watchlist in test

[samford]

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The existing watchlist test in test/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.

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For what it's worth, the test used to pass even if there was a watchlist file when the default location was ~/.brew_livecheck_watchlist. This is because the home directory is HOMEBREW_PREFIX/Library/Homebrew/test/ in tests and there isn't a .brew_livecheck_watchlist file there. The default livecheck watchlist path changed in #15787 such that it's now available in the testing environment, so now we encounter this issue.