[backport] SI-7498 ParTrieMap.foreach no longer crashes

Previously, the `split` method of the `ParTrieMap` iterator threw
an exception when splitting a splitter that iterated over nodes
whose hash codes collide.
This was due to reusing the iterator of the list of colliding keys
rather than creating a new splitter.

This commit changes the `subdivide` method to create a new
iterator using the factory method of the current trie map
iterator rather than returning a `LinearSeqLike` iterator.
Merge pull request #2644 from retronym/ticket/7498-backport

[backport] SI-7498 ParTrieMap.foreach no longer crashes
Add support for JUnit tests

Add `test.junit` ant target that compiles and runs JUnit tests
found in `test/junit` directory.

Add `scala.tools.nsc.SampleTest` that demonstrates working
testing infrastructure.
Add Eclipse project for JUnit tests.
Merge remote-tracking branch 'scala-2.10.x/junit' into master
Move WeakHashSetTest to junit tests.

The WeakHashSetTest was written as unit test but put into partest's
`run` category as we were missing direct unit testing support.
That got fixed so moving the test now.
Merge pull request #2657 from gkossakowski/junit-master

Merge JUnit support into master