Added ScalaTest support to core build.xml

The old JUnit tests are still present, but "ant test" and "ant fulltest" default
to the (pending) ScalaTest ones. To run the ScalaTest tests with Cobertura, execute
the following:

ant -Dscalatest.libs=./lib -Dwith.cobertura=/usr/share/java/cobertura.jar fulltest

The scalatest.libs property must point to a directory containing scala-compiler.jar,
scala-library.jar and scalatest.jar.