Ensure xattr checksum caching is tested

[malmond77]

Context: https://github.com/rpm-software-management/librepo/pull/234/files/2743583e444d745526e9bb8fad249ce6ea08e0f9#r594141462

@m-blaha highlights a bug in the test suite where xattrs are tested. The temporary files often end up in /tmp which on some systems can be tmpfs, which doesn't support user attributes.

Two directions are possible:

1. Force tests to use non tmpfs, which improves the odds of getting a filesystem that supports xattrs with user attributes
2. Mock the xattr calls, removing the hard dependency on real xattr support