Allow running doctests via rust.doctest() #17416

Job	Run time
Arch Linux	20m 24s
CUDA (on Arch)	7m 16s
Fedora	17m 42s
Gentoo	22m 34s
OpenSUSE	16m 20s
Ubuntu Bionic	15m 11s
Arch / PyPy	20m 2s
Ubuntu Rolling (gcc, g++)	19m 16s
Ubuntu Rolling (--unity=on -Ddefault_library=static, --no-unittests, gcc, g++)	13m 29s
Ubuntu Rolling (-Ddefault_library=both, --no-unittests, gcc, g++)	13m 33s
Ubuntu Rolling (clang, clang++)	22m 28s
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 15s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, --unity=on, gcc, g++)	13m 33s
	3h 50m 43s

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