Use Junit RunListener to make a better summary report for tests

[smarter]

See http://junit.org/junit4/javadoc/latest/org/junit/runner/notification/RunListener.html, this can be passed to https://github.com/sbt/junit-interface using the --run-listener= option.

[DarkDimius]

• it would be nice to restore context leak detection. It used to rely on the same mechanism https://github.com/lampepfl/dotty/pull/119