selftests/cgroup: use bash in test_cpuset_v1_hp.sh

The script uses non-POSIX features like `[[` for conditionals and hence
does not work when run with a POSIX /bin/sh.

Change the shebang to /bin/bash instead, like the other tests in cgroup.

Signed-off-by: Bharadwaj Raju <bharadwaj.raju777@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>