Don't package tests directory

[lambda]

The setuptools.find_packages command will by default find all Python packages starting at the root directory, which in this case includes the tests directory. That means that a tests package gets installed into your site-packages when you install this. That can then cause problems if a user is using pytest --import-mode=append tests in order to test an installed version of a package that they are working on that depends on polarion, as that will put the users tests directory at the end of sys.path, and instead the installed polarion package tests directory will be searched.

I don't believe there is any reason to ship the tests with the package, so limit the included packages to only include what is necessary when building the package.