Add support for JUnit 4.12 private field rename

As part of [this JUnit commit](https://github.com/junit-team/junit/commit/df00d5eced3a7737b88de0f6f9e3673f0cf88f88), the private field `fTestClass` was renamed to `testClass`. I found a related discussion [here](https://github.com/junit-team/junit/issues/960).

This fix should add support for using gwtmockito with JUnit 4.12+.