[atf] add test for class load override

Class loader sometimes didn't work and it didn't get caught with
existing tests as it only appears when we are dealing with support lib.

Adding unit test that can catch such cases earlier going forwards.

Bug: 180049462
Test: Passes locally
Change-Id: I3150970f45d5a42ed6ce9e4453c7bc63d6160ee5
2 files changed