test_runner: fix coverage report when a directory is named file #2377

Job	Run time
Build slim tarball	1m 31s
x86_64-darwin: with shared libraries	44m 46s
aarch64-linux: with shared libraries	22m 18s
x86_64-linux: with shared libraries	27m 0s
aarch64-darwin: with shared libraries	25m 35s
	2h 1m 10s

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