ipatests: Fix a call to run_command with wildcard
    
    The test is calling run_command with a list of arguments:
    run_command(['rm', '-f', paths.CERTMONGER_REQUESTS_DIR + '/*'])
    but this format does not support shell expansion.
    
    Replace with a str parameter:
    run_command('rm -fv' + paths.CERTMONGER_REQUESTS_DIR + '/*')
    
    to make sure all the files in the directory are actually removed.
    
    Fixes: https://pagure.io/freeipa/issue/8506
    Signed-off-by: Florence Blanc-Renaud <flo@redhat.com>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>