mtest: list the first N log lines rather than the last N #18005

Job	Run time
Arch / PyPy	22m 31s
Arch Linux	21m 17s
CUDA (on Arch)	10m 16s
Ubuntu Rolling (gcc, g++)	22m 19s
Fedora	21m 9s
Gentoo	25m 20s
Ubuntu Rolling (--unity=on -Ddefault_library=static, --no-unittests, gcc, g++)	15m 53s
OpenSUSE	18m 43s
Ubuntu Rolling (-Ddefault_library=both, --no-unittests, gcc, g++)	35m 14s
Ubuntu Bionic	15m 40s
Ubuntu Rolling (clang, clang++)	26m 0s
Ubuntu Rolling (--unity=on, --no-unittests, clang, clang++)	18m 50s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, 0, gcc, g++)	14m 3s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, --unity=on, gcc, g++)	14m 10s
	4h 41m 25s

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