dev-cmd/livecheck: guard watchlist test

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 adds an `if` condition to the test, to ensure that it only runs
when a livecheck watchlist file doesn't exist.

Library/Homebrew/test/dev-cmd/livecheck_spec.rb

@@ -19,7 +19,19 @@
       .and be_a_success
   end
 
-  it "gives an error when no arguments are given and there's no watchlist", :integration_test do
+  # When a watchlist file is present, livecheck parses the formula/cask names
+  # in the file and ignores any that aren't available. Since formulae and casks
+  # aren't available in the testing environment, the watchlist file will be
+  # treated as empty and this produces an `Invalid usage: No formulae or casks
+  # to check.` error instead. As such, this test will only pass when a
+  # watchlist file isn't present in the testing environment.
+  #
+  # Also note, any older `~/.brew_livecheck_watchlist` file won't be accessible
+  # from this test (as the home folder is `HOMEBREW_PREFIX/test/`), so we don't
+  # have to guard against its existence.
+  it "gives an error when no arguments are given and there's no watchlist",
+     :integration_test,
+     if: !File.exist?(Homebrew::EnvConfig.livecheck_watchlist) do
     expect { brew "livecheck" }
       .to output(/Invalid usage: A watchlist file is required when no arguments are given\./).to_stderr
       .and not_to_output.to_stdout