test_runner: support mocking non-existent modules #4332

Job	Run time
Build slim tarball	2m 0s
aarch64-linux: with shared libraries	17m 52s
aarch64-darwin: with shared libraries	18m 4s
x86_64-darwin: with shared libraries	28m 34s
x86_64-linux: with shared libraries	22m 10s
	1h 28m 40s

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