`Minitest.plugin_rails_init` sets `Minitest.backtrace_filter` to
`Rails.backtrace_cleaner` right before tests are run, overwriting the
value set in test_helper.rb.
`Rails.backtrace_cleaner` silences backtrace lines that do not start
with `Rails.root` followed by e.g. "lib/" or "test/". Thus when
`Rails.root` is a subdirectory of the project directory -- for example,
when testing a plugin that has a dummy app -- all lines of the backtrace
are silenced.
This commit adds a fallback such that when all backtrace lines are
silenced, the original `Minitest.backtrace_filter` is used instead.
Additionally, this commit refactors and expands existing test coverage.
* 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)
