Re: [PATCH v3] t7611: replace test -f with test_path_is* helpers

> Replace `test -f` and `test ! -f` with `test_path_is_file` and
> `test_path_is_missing` for better debuggability.
>
> While `test -f` ensures that the file exists and is a regular file,
> `test_path_is_file` provides clearer error messages on failure. On the
> other hand, `test ! -f`, used to check either the absence of a regular
> file or the presence of any other filesystem object, but looking at
> them in the test individually, all of them should've said `test ! e`,
> i.e. "there shouldn't be anything at given path on filesystem."
> Replaced these cases with `test_path_is_missing` for better
> debuggability.