Add a command line flag that prints logged messages even if the test succeeds

Sometimes it's useful to print out the logs even if a test succeeds for debugging purposes. It would be useful to have a flag that can be passed as a command line argument to `cabal test` that forces all of the `logDoc` messages to always be printed.

Start by tracing how `cabal test` arguments reach the test runner and where `logDoc` messages are currently emitted or suppressed. Add a command-line flag that enables those messages for successful tests, then verify that normal output remains unchanged without the flag and that successful tests print their logs when it is enabled.

Written by the indexing model from the issue text.