Silence Minitest for plugin tests.

Fixes https://github.com/rails/rails/issues/17340.

Use Minitest::BacktraceFilter instead of removing all silencers. This
will allow the backtrace for all libraries in the plugin to be shown
while removing noise generated by Minitest's backtrace.
Merge pull request #17867 from tgxworld/silence_minitest_plugins

Silence Minitest for plugin tests.