Make "test" optionally show a diff of unmatching cases

[Thomas-Frew]

There are many scenarios where I am testing a problem, and would love to see the difference between what my code generated, and what the answer should be. Obviously, this isn't feasible for all problems. Adding an optional tag to do this for problems with one (or very few) lines of output would save me tons of time.

[rayokamoto]

Yeah, this is a good suggestion.

I was thinking of a --diff flag to optionally show a diff (because on by default would be real annoying if the differences were hundreds, if not thousands of lines long).

We can output the diff to stdout rather than stderr so it can be piped into a file for further viewing as well (and it won't collide with diagnostic output from the CLI).