`cabal-validate`: "Keep going" mode

If any tests fail, the whole `./validate.sh` script exits early. This is useful for quick feedback, but can make it annoying to get a grasp on the full set of errors when making larger refactors. I have to fix all the errors in a test suite before I can get to the next one to even see if it errors.

**Proposal:** There should be a `--keep-going` or `--no-fail-fast` option which continues even if a test suite fails. The exit code should still be set the same (any test failures lead to a non-zero exit code) and a summary of which steps passed or failed should be printed at the end of the output.