Strict verification of resource deletion in TemporaryFolder rule

PR that adds support of strict verification of deleted resources when using `TemporaryFolder` rule #1001
`IllegalStateException` doesn't sound right to me because it "signals that a method has been invoked at an illegal or inappropriate time"

I think we should either throw `AssertionError` or `RuntimeException`. In either case, we could introduce a custom exception in the future.

Let's go with `AssertionError`. I have a sight preference for using `fail()` vs. `assertTrue()` here