Add a simple reproducibility test command. #16688

Job	Run time
Arch / PyPy	19m 56s
Arch Linux	19m 43s
CUDA (on Arch)	8m 2s
Fedora	20m 13s
Ubuntu Rolling (gcc, g++)	20m 15s
Gentoo	22m 6s
Ubuntu Rolling (--unity=on -Ddefault_library=static, --no-unittests, gcc, g++)	13m 47s
OpenSUSE	16m 14s
Ubuntu Bionic	15m 45s
Ubuntu Rolling (-Ddefault_library=both, --no-unittests, gcc, g++)	13m 53s
Ubuntu Rolling (clang, clang++)	23m 33s
Ubuntu Rolling (--unity=on, --no-unittests, clang, clang++)	15m 20s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, 0, gcc, g++)	14m 4s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, --unity=on, gcc, g++)	14m 4s
	3h 56m 55s

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