Provide a TestUtils class in the main artifact

The `TestUtils` class was already forked in `ij1-patcher` and `MiniMaven`. The latter also had enhancements that more users might want to benefit from. Let's port them back into `scijava-common` and drag the `TestUtils` out of `src/test/java/`'s shadows into `src/main/java/`'s lime light.

This addresses #63.