test: fix hang when running tests that need parsing with `--interactive` #17194

Job	Run time
Arch / PyPy	19m 40s
Arch Linux	21m 26s
CUDA (on Arch)	7m 4s
Ubuntu Rolling (gcc, g++)	20m 12s
Fedora	18m 8s
Ubuntu Rolling (--unity=on -Ddefault_library=static, --no-unittests, gcc, g++)	13m 31s
Gentoo	22m 4s
Ubuntu Rolling (-Ddefault_library=both, --no-unittests, gcc, g++)	13m 57s
OpenSUSE	16m 42s
Ubuntu Rolling (clang, clang++)	22m 22s
Ubuntu Bionic	49s
Ubuntu Rolling (--unity=on, --no-unittests, clang, clang++)	15m 14s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, 0, gcc, g++)	13m 46s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, --unity=on, gcc, g++)	13m 32s
	3h 38m 27s

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