Strict verification of resource deletion in TemporaryFolder rule

PR that adds support of strict verification of deleted resources when using `TemporaryFolder` rule #1001
Sorry for not commenting on this earlier (really hard to see comments on anything but the most recent diff in GitHub when you squash commits).

Anyway... I don't think many people will want their tests fail when the temporary folder cannot be deleted *and* allow some tests to pass if they prevent the temporary folder from being deleted. So I'm fine not having `TemporaryFolder.assureDeletion()`