Add JUnit support

As discussed on [scala-internals](https://groups.google.com/d/msg/scala-internals/qPbD5d15kvA/p0Nn2-YKvtkJ) adding JUnit support is probably a good idea. We write unit tests in partest which wasn't designed for that.

This PR adds a basic infrastructure for running JUnit tests:

  * an Ant task `test.junit` that runs all JUnit tests defined in `test/junit` directory
  * an Eclipse project definitions that allows us one to import all the tests to Eclipse

There will be a follow up PR for master which merges this PR and migrates WeakHashMap test to be a junit test.