Print overall tests duration per thread if in debug mode

[csoltenborn]

Sometimes parallel test execution takes longer than expected. On reason might be that tests are distributed to the different threads in a sub optimal way (note that this is expected to happen before all tests have been run at least once). To debug such issues (or to find out that instead, the executed tests themselves have unpredictable durations), print out the duration of each executing thread if in debug mode.