AbstractCacheCleanupTest.groovy

Delete empty parent dirs of deleted cache entries
Prior to this commit parent directories of deleted cache entries were
not checked whether they're eligible for deletion. Now, parent
directories that are now empty are checked up to the base directory.

For the artifact transforms cache the following directories marked with
(+) are now also deleted once all subdirectories (*) have been deleted:

- files-1.1
- artifact-a (+)
- hash 1 (*)
- hash 1.1
- ...
- hash 2 (*)
- ...
- ... (+)

For the artifact cache:

- modules-2
- files-2.1
- groupId (+)
- artifactId (+)
- version (+)
- hash1 (*)
- file
- hash2 (*)
- file
- resources-2.1
- 0 (+)
- hash 1 (*)
- hash 2 (*)
- ...
- ... (+)

Resolves gradle/gradle-private#1339.