test: fix test-fs-open
If there is a file 'path' in the root dir, the test will fail with the 'ENOTDIR' instead of 'ENOENT'. Change path to something more unlikely.
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
documentation is changed or added -
commit message follows commit guidelines