`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.