test: improve test coverage for inpath() function

The inpath() function appears to not be getting tested on windows:

Coverage details: https://app.codecov.io/gh/intel/cve-bin-tool/blob/main/cve_bin_tool/util.py#L112

It looks like there is a test in test/test_util.py already; is there some reason it's not triggering?  Does the test need a change, or does the code need a change?  Or is the coverage not reporting it for some reason?
