Add a simple reproducibility test command. #16605

Job	Run time
Arch Linux	19m 44s
Arch / PyPy	19m 27s
CUDA (on Arch)	7m 31s
Fedora	19m 37s
Ubuntu Rolling (gcc, g++)	19m 16s
Gentoo	22m 10s
Ubuntu Rolling (--unity=on -Ddefault_library=static, --no-unittests, gcc, g++)	13m 28s
OpenSUSE	15m 58s
Ubuntu Rolling (-Ddefault_library=both, --no-unittests, gcc, g++)	13m 47s
Ubuntu Bionic	14m 54s
Ubuntu Rolling (clang, clang++)	22m 17s
Ubuntu Rolling (--unity=on, --no-unittests, clang, clang++)	15m 28s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, 0, gcc, g++)	13m 20s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, --unity=on, gcc, g++)	13m 27s
	3h 50m 24s

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