spec: Default to not shipping tests on RHEL > 7
    
    We weren't shipping it in RHEL7 either. It also brings in a dependency
    on Python, which is tricky there. (And anyway
    `gnome-desktop-testing-runner` isn't even packaged there).
    
    Make it a bcond though so one can still build it if one desires. (And
    symmetrically, to *not* build it on Fedora if one desires).