Proper cleanup after finishing global-showdef test

The temporary foo directory with classes created by global-showdef.scala was not deleted after finishing a test. When I was running tests from Scala project root, this directory was created there and was visible when running git status.

I just forced the creation of a foo directory inside global-showdef-run.obj - so it's properly deleted after finishing the test.
This should be equivalent, but is more consistent with other tests.

```scala
     val command              = new CompilerCommand("-usejavacp" :: "-d" :: testOutput.path :: args.toList, settings)
```