linux

Allow running doctests via rust.doctest() #17401

Job	Run time
Arch Linux	19m 16s
CUDA (on Arch)	7m 32s
Arch / PyPy	21m 26s
Fedora	16m 44s
Gentoo	21m 3s
OpenSUSE	15m 40s
Ubuntu Rolling (gcc, g++)	18m 39s
Ubuntu Bionic	13m 22s
Ubuntu Rolling (--unity=on -Ddefault_library=static, --no-unittests, gcc, g++)	14m 9s
Ubuntu Rolling (-Ddefault_library=both, --no-unittests, gcc, g++)	13m 52s
Ubuntu Rolling (clang, clang++)	21m 35s
Ubuntu Rolling (--unity=on, --no-unittests, clang, clang++)	15m 40s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, 0, gcc, g++)	13m 33s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, --unity=on, gcc, g++)	13m 26s
	3h 45m 57s

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