Better testing infrastructure for metadoc cli

[olafurpg]

Follow up on #55 (comment)

An idea testing infrastructure makes it easy to

• add new tests
• update tests when expected behavior changes
• review diffs from test

I'm personally a big fan of "expect tests" (see https://blog.janestreet.com/testing-with-expectations/), which are similar to partests in scala/scala.

Example expect tests in scalameta: https://github.com/scalameta/scalameta/blob/15a0187501dc8e5a6fe06d0e002ce1d3340968dc/scalameta/semanticdb-scalac/src/test/scala/scala/meta/tests/SemanticSuite.scala#L6

I'd love to setup something similar for metadoc once I finally get through with this scalafix 0.5.0 release.