fix coverage #908

Job	Run time
ubuntu_clang (Release, OFF, ON)	3m 25s
ubuntu_clang_for_liburing (Release, OFF, ON)	2m 58s
ubuntu_clang (Release, OFF, OFF)	3m 15s
ubuntu_clang_pb (Release, OFF, ON)	2m 53s
ubuntu_clang_for_liburing (Debug, OFF, ON)	3m 10s
ubuntu_clang (Debug, OFF, ON)	3m 7s
ubuntu_clang10 (Release)	2m 33s
ubuntu_clang (Debug, OFF, OFF)	3m 4s
ubuntu_clang_pb (Release, OFF, OFF)	2m 52s
ubuntu_clang10 (Debug)	2m 35s
ubuntu_clang_pb (Debug, OFF, ON)	2m 59s
ubuntu_clang_pb (Debug, OFF, OFF)	3m 0s
	35m 51s

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