Brew: Improve Homebrew/brew code coverage

[MikeMcQuaid]

It would be good to increase our code coverage. This can be done by adding integration tests (only) for missing cmd and dev-cmd commands (e.g. https://codecov.io/gh/Homebrew/brew/src/master/Library/Homebrew/cmd/diy.rb), figuring out why some coverage is currently missing (e.g. https://codecov.io/gh/Homebrew/brew/src/master/Library/Homebrew/dev-cmd/ruby.rb despite https://github.com/Homebrew/brew/blob/master/Library/Homebrew/test/dev-cmd/ruby_spec.rb) and adding unit tests for files with low coverage (e.g. https://codecov.io/gh/Homebrew/brew/src/master/Library/Homebrew/hardware.rb).

[alexbostock]

If I make more contributions like #4915, is it better to add lots of tests in the same pull request or make lots of separate pull requests?

[reitermarkus]

@alexbostock, many small PRs are easier to review than a huge one.

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.

[MikeMcQuaid]

I think this is now good enough (>70%) that we don't need an open issue.