BACKTRACE environment variable to show unfiltered backtraces.

We used to support the `BACKTRACE` environment variable but when
we switched to MiniTest it got removed: f9382cd7948

This commit adds back the functionality to show the unfiltered backtrace when needed.
This also works when you run your tests with `rake`:

* `BACKTRACE=1 bin/rake test`
* `BACKTRACE=1 ruby -Itest ...`