test_runner: support mocking non-existent modules #4499

Job	Run time
Build slim tarball	1m 54s
aarch64-linux: with shared libraries	17m 42s
aarch64-darwin: with shared libraries	18m 25s
x86_64-darwin: with shared libraries	30m 9s
x86_64-linux: with shared libraries	21m 51s
	1h 30m 1s

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