For test failures, should we show the actual and expected values?

[glennj]

With a few small changes to the exercism/test library, we can achieve this.

Some example output:

@axtens @softmoth
Is it a good idea?
Do you like the output (particularly, opinions about the angle quotes)?

[axtens]

Chevrons are cool and are used in other places as "really obvious quotation marks". In this PR what's not to like?

[softmoth]

It's great! I can imagine if the expected value is very long, it might be nice to have truncated, but that can be dealt with at a later time if the need crops up.

[axtens]

Which, when you get to that, should be a middle truncation e.g. tru..ion