build: Exclude tests from being recognized as package

[aragon999]

I maintain the AUR package of this tool:

Today there was a bug report, that two packages have conflicting files, one beeing python-csv-diff: https://aur.archlinux.org/packages/python-csv-diff#comment-896123

This change will in the future not package the tests directory, as it is not recognized as package anymore. I included this patch in the AUR package.

Thank you for the tool.