tfragment_gc test: make more robust
tfragment_gc test: make more robust

dom96/Nim,dom96/Nim,dom96/Nim,dom96/Nim,dom96/Nim,russpowers/Nim,dom96/Nim,russpowers/Nim,russpowers/Nim,dom96/Nim,russpowers/Nim,russpowers/Nim,russpowers/Nim,russpowers/Nim