Strict verification of resource deletion in TemporaryFolder rule

PR that adds support of strict verification of deleted resources when using `TemporaryFolder` rule #1001
The only other things that comes to mind is to use an `AssertionError` or even `assertTrue("Unable to clean up temporary folder " + folder, isDeletionAssured())`. What do you think?

On the other hand, I could live with IllegalStateException, too.