Clean up testsuite intermediate files #16609

Job	Run time
Arch Linux	19m 32s
Arch / PyPy	19m 23s
CUDA (on Arch)	7m 31s
Fedora	19m 43s
Ubuntu Rolling (gcc, g++)	19m 28s
Gentoo	21m 37s
Ubuntu Rolling (--unity=on -Ddefault_library=static, --no-unittests, gcc, g++)	13m 23s
OpenSUSE	15m 23s
Ubuntu Rolling (-Ddefault_library=both, --no-unittests, gcc, g++)	13m 46s
Ubuntu Bionic	14m 40s
Ubuntu Rolling (clang, clang++)	22m 23s
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 6s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, --unity=on, gcc, g++)	13m 47s
	3h 49m 22s

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