`cabal-validate`: Skip "low-value" tests by default?

Some tests, like the VCS tests (see #10561), take a long time to run and rarely catch bugs. We want these to run in CI, and we want there to be an option to run them, but we don't necessarily want to run them by default.

On the other hand, it can be very annoying if the default test command doesn't actually run all the tests, and it can lead to unexpected failures in CI.