Move partest out to its own repo: scala/scala-partest.

As partest is now resolved from maven, `test/partest` uses `ant test.suite.init`
to determine the classpath (serialized to build/pack/partest.properties)
that's necessary to run `scala.tools.partest.nest.ConsoleRunner`.

Thus, partest gets exactly the same classpath, whether run from
the command line through `test/partest` or via `ant test`.

The version of partest we're using is specified by
properties defined in `starr.number`. Currently:

```
scala.binary.version=2.11.0-M4
partest.version.number=1.0-RC1
```

NOTES:
  - The version of Scala being tested must be backwards binary compatible with
    the version of Scala that was used to compile partest.
  - Need scalacheck on classpath for test/partest scalacheck tests.
  - Remove atrophied ant tests (haven't been run/changed for at least two years
    I checked 81d659141a as a "random" sample).

review by everyone (@gkossakowski, @jamesiry, @jsuereth, @lrytz, @paulp, @retronym, @soc)