ipatests: Specify shell implementation
    
    The shell command line options and parameters used there are bash-
    specific. This results in an error on attempting of running
    'ipa-run-tests' on systems where '/bin/sh' is pointing to another
    shell, for example, dash on Ubuntu.
    
    Fixes: https://pagure.io/freeipa/issue/8101
    Signed-off-by: Stanislav Levin <slev@altlinux.org>
    Reviewed-By: Sergey Orlov <sorlov@redhat.com>