Test failure: report better error message when details are different only by whitespace

[jfmengels]

In the test failures, we report an issue when details are different than expected. However, when the difference is only whitespace, it can be really hard to track down the problem.

We already do an improved diff (with colors) when it comes to checking the fixed code, and I'm thinking we could do the same for the details as well. Maybe we could / should display the details as Elm lists too, so that they're easier to copy over to the test.