* Don't use :: for class methods, we don't do that elsewhere.
* Don't install a needless method on minitest. Prefer assigning the
reporter anyway as that's what minitest does internally.
* Don't bother opting out when the reporter ain't a Minitest::CompositeReporter.
It's hardcoded: 005a3ba42c/lib/minitest.rb (L125)
And overrides have to create delegate reporters:
1018b1b42f/lib/minitest/minitest_reporter_plugin.rb (L72)
