add new test which covers rpmdb program
    
    Recent SELinux policy brought new types like rpmdb_exec_t and rpmdb_t,
    which are assigned to the rpmdb program and to the rpmdb process.
    Because we don't have any test coverage for these types, I wrote a basic
    test which runs the rpmdb-rebuild service in default configuration and
    looks for any SELinux denials.