Add debug logging for test collection

[amandelpie]

Now we could not track the progress of test handling.

I suggest adding some logger calls with DEBUG level for the tracking what unit tests were discovered and visited, probably processed modules and some stats à la (100/11000 were processed)

[amandelpie]

We need to create the instance of FileTreeWalk manually, it has some events (onEnter, onLeave) where we could track leaving the file/directory