[cosgroveb]

Some CI test views read only JUnit XML (Harness reads JUnit XML or TRX),
so checkstyle output never shows up there.

JUnit has no concept of a lint finding. The formatter copies the mapping
rubocop --format junit uses: one testcase per runner and changed file,
tests counts files, failures counts messages. This makes this a bit of
a "square-peg in a round hole" kind of thing which is similar to Rubocop's
own JUnit runner. It would be better for me to use checkstyle! But alas...

Two things:

1. A runner can show a phantom pass for a file it never checks
   (pronto-rubocop on a .xml file). Runners filter files internally,
   and formatters can't see that filtering. RuboCop has the same gap one
   level down: rubocop --format junit a.rb lists Bundler/GemFilename
   as passing.
2. Runners#active_runners is now public, so the formatter applies the
   same runners, skip_runners, max_warnings and exclude rules
   instead of copying them.

An optional runner hook (say Runner#inspected_patches, defaulting to
every patch) would solve the phantom-pass thing but I felt that a smaller, more
reviewable change would be best for now. I'm happy to follow up if this sounds
like a reasonable thing to change.

I didn't see an AI assistance policy and it's understandably a hot topic right
now so in the interest of openness... I did generate the code using claude
code. I reviewed it carefully through a few iterations and tried to hew as
closely to project standards. Any mistakes are my own.