Add support for colors to testsuite

The `cabal-testsuite`'s output is imho difficult to comprehend visually, especially when more than a single test fails.

Maybe adding color support (either via `ansi-terminal` or by using the few ANSI escape sequences we need directly) to highlight the structure of the output (e.g. to make it more obvious where the output for one test-case ends, and the output for another starts) may go a long way.

Color support should ideally detect whether it's being directed at a console or directed to a file (and then disable it), and/or give the ability to control color output via a `--color` flag.