Add support for running a specific Junit test/method.

Unless I really missed something, there is currently no way to run a
specific Junit test (class or method). Running the whole thing is going
to get worse and worse since Junit is supposed to be the preferred way,
so here it goes:
```sh
# will run all methods in a test file
ant test.junit -Dtest.class=scala.WhateverTest

# will run a specific method (just an alias for the one below)
ant test.junit -Dtest.class=scala.WhateverTest -Dtest.method=mymethod

# will run several methods (comma separated)
ant test.junit -Dtest.class=scala.WhateverTest -Dtest.methods="foo,bar"
```
`test.method(s)` without `test.class` is just ignored (all tests are
run).