[TEST] Beef up InternalEngineTest and remove bogus timeouts
Ban java.io.File in tests.

Restrict use of java.io.File to 5 methods (excluded), but otherwise ban.
This is a prerequisite to do any mocking here.

I don't try to do any heavy cleanup on these tests, I am not familiar with them.
So this is mostly a rote straightforward conversion.

Closes #8836