Fixes Testkit implicit sender bug & FullDebugConfig-bug

When creating TestKit, before setting the `TestActor` as implicit sender I checked if `InternalCurrentActorCellKeeper.Current` was set. If it wasn't then `TestActor` was set.

The check messed up tests that were run consecutively, as the second test would not set `TestActor` as sender if it was started on the same thread that the previous test ran on.
This bug has not been seen before, since practically all of the internal tests defines sender explicitly.
This commit removes that check.

Also, when using `FullDebugConfig` it tried to log using `TestEventListener`. This is already done by TestKit as default, so it has been removed.