Silence Minitest for plugin tests.

Fixes https://github.com/rails/rails/issues/17340.

Rails::BacktraceCleaner's silencers are removed to obtain the full backtrace for all libraries included in the plugin which can be helpful when debugging errors. However, we will silence traces for Minitest as it adds noise to the backtrace.

cc/ @rafaelfranca  @chancancode 
Can we add `Minitest::BacktraceFilter`?