Separate linting from test tasks

[wasabigeek]

Early on, when running bundle exec rake test, I mistakenly assumed that if only linting errors showed, then the tests were fine. I think separating the linting from testing would prevent this newbie mistake 😄 and also make it easier for someone who was doing small steps (e.g. https://github.com/freerange/mocha/pull/552/commits/9361f8e6d8078ed844cfe0ac9235274a3e231035#diff-d9dbcb54883fff83b955f57c445b2c7c0862b1ab71378f266096443cb652c136R40 fails the linting).

I think CI-wise it might make sense to run linting separate from tests (so at a glance one can see whether linting/tests are failing) but have not done so in this PR. Does Rubocop need to be run for every Ruby version?