mailcatcher: fix missing UI assets

[p-linnane]

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Continuing #174319.

[p-linnane]

@sj26 The issue we are running into here is that the Rakefile is running tests requiring RSpec unconditionally. This is not something we do for our packages in Homebrew/core. Our options here would be to patch the formula (like I did for $LOAD_PATH, since mail_catcher/version was not being found) to remove the RSpec tests, or for tests to not be run by default in the Rakefile.