Improved metrics: Measure and report test coverage

[dnfield]

It would be great if pub encouraged package authors to write tests for their code. If a repository has a lcov file, or perhaps a link to coveralls or some similar service, pub could calculate test coverage into the score for the package.

[jonasfj]

If we ran tests I think this would make sense, but without running tests it's just a vanity thing (as we can't tell how fresh the results are).

Don't get me wrong, running tests and publishing test coverage metrics would be awesome.

[sigurdm]

Still valid

[benthillerkus]

Instead of running tests, a first step could be to just display how many test cases there are or how many LOCs the test/ folder has