ipatests: Properly kill gpg-agent
    
    There is a race condition exposed in 'test_gpg_asymmetric'.
    The teardown of 'tempdir' fixture and gpg-agent being called
    from the teardown of 'gpgkey' fixture could simultaneously
    remove the gnugpg's socket files.
    
    This results in an error like:
    ```
    
    ================= ERRORS ===================
    _ ERROR at teardown of test_gpg_asymmetric __
    ...
    
    >  os.unlink(entry.name, dir_fd=topfd)
    E  FileNotFoundError: [Errno 2] No such file or directory: 'S.gpg-agent.extra'
    
    /usr/lib64/python3.7/shutil.py:450: FileNotFoundError
    
    ```
    
    The problem is that the agent is not terminated properly.
    Instead, gpgconf could be used to kill daemonized gpg-agent.
    
    Related: https://pagure.io/freeipa/issue/7989
    Signed-off-by: Stanislav Levin <slev@altlinux.org>
    Reviewed-By: Christian Heimes <cheimes@redhat.com>