Separate RPM installability test from rpm_tests
    
    tests/rpm_tests mostly inspects the built rpms, which neither requires
    any privileges nor apply any changes to the system.
    
    `InstallRPMTestCase` stands out, as it assumes that the installation
    happens in mock (which is not true any more when testing in containers
    or a local build environment). It also requires a lot of code/mechanics.
    
    Move the installability test into the `make test-rpm` top-level rule
    (which is the one called by Jenkins), and drop `InstallRPMTestCase`.
    
    Cherry-picked from master commit 0bf0c04e.
    
    Related: rhbz#1885635