Add ability to run multithreaded testing FLAC files in directory tree about flac2all HOT 2 OPEN

All the code for multithreaded testing is already in the stable version of 
flac2all. 
It used to officially support it, but it was buggy, and there was no way of 
making a proper log/summary of files that were not OK. 

The feature is planned to be implemented properly in v4, with the ability to 
print a report of which files are not OK, but for now, you can try running it 
on the stable version with command "test" rather than mp3/ogg/aac.

Let me know if it works! This part of the code is not up to the same standard 
as the rest.