TDF: failure in test-reports

Report sometimes misses one entry in the count, resulting in test failures with this output diff:

-fx1:pass=6all=10--60.000%

-fx2:pass=4all=6--66.667%

+fx1:pass=5all=9--55.556%

+fx2:pass=4all=5--80.000%

 fx3:pass=1all=4--25.000%

I am not sure about the cause of the problem, so far I was not able to reproduce it locally and clang's thread sanitizer does not pick up anything relevant. To be investigated.