Use java-diff-utils for diffing in partest.

We now use the unified diff format, hence the updated check files.

It's not clear to me how partest's classpath is managed,
but the approach in this commit works for the ant task and script invocation.
The diffutils jar is injected in the parent classloader.
Strip version suffix from diffutils.
Merge pull request #2330 from adriaanm/partest-difftools

Use java-diff-utils for diffing in partest.