Feature: fail_with matcher

[sascha-wolf]

It would be useful to have a fail_with matcher similar to RSpecs fail_with.

Sometimes we have to write some more complex tests which expect certain underlying behaviour from the tested code which is hard to cover with the existing expectations. On some occasions a custom assertions helps but on others such an assertion would be overkill.

Having a simple option to fail a test in an expected manner without raising an arbitrary error would increase test readability a great deal.

[antonmi]

Hi, @zeeker !
Interesting idea!
Thank you!