Unit Tests For SquashBacktrace.populatenestedexceptions()

Hi,

I have added some unit tests for `SquashBacktrace.populateNestedExceptions()`. I see that there is already a set of comprehensive test cases under `SquashEntryTest`. But I think separating out the utility test cases into their own test classes will help to improve readability and maintainability. 

I also added a private method named `nestedExceptionsWithDepths()` to help create exceptions with N-level deep nested causes.