Show location of test failures

While exception messages show a the file name/line number where the exception was thrown, actual test failures (e.g. `refute true`) were not displaying the location of the exception.

To expose the problem, I ran the bad test gallery with stock minitest pride output. The `__FILE__` and `__LINE__` where the failed assert occurred were showing up. However, when I switched to `Minitest::Reporters::DefaultReporter`, the failure location was gone.

I also added the ability to easily run the galleries with minitest pride output as a way of testing the formatting of `DefaultReporter` against stock minitest in the future.