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>