Groovy project doesn't build on Windows

Builds on Windows fail for two reasons. First the line to create extTestDir is creating a path consisting of two full paths concatenated together. The second drive letter is seen as an illegal character:

private static final File extTestDir = new File(System.getProperty("user.dir"), TestHelper.makeTestDataDirectory(DependencyGrabberTest.class));

Second, when it comes time to delete the directory it is locked. This is because some instances of JarFile are created on it but not closed.