Minitest 5 (continuation of #358)

There is no progress in https://github.com/Shopify/liquid/pull/358, but I would like to merge it. This PR addresses the code review comments.

@arthurnn @jasonhl, could you review this? Please read the original PR description first so you know what's up with the whole global filter thing.

cc @dylanahsmith @ktdreyer

Closes #358 
Nope, this should work with both versions, I think. There is code for that in test_helper.rb.