[FIX] import unit_threaded only in unittests
Hi,
Thanks a lot for this simple library for console colors)
This is needed to avoid errors, when this library is used with other test runnner. Example of error that is fixed by this commit is: https://github.com/katyukha/Odood/actions/runs/4565738153/jobs/8057274799
PS: Please, release new version on dub after this fix merged. PS2: May be it have sense to use something more lightweight than unit-threaded for unittests?