cgroup: selftests: Move memcontrol specific helpers out of common cgroup_util.c

cgroup: selftests: Move memcontrol specific helpers out of common cgroup_util.c

Move a handful of helpers out of cgroup_util.c and into test_memcontrol.c
that have nothing to with cgroups in general, in anticipation of making
cgroup_util.c a generic library that can be used by other selftests.

Make read_text() and write_text() non-static so test_memcontrol.c can
use them.

Signed-off-by: James Houghton <jthoughton@google.com>
Acked-by: Michal Koutný <mkoutny@suse.com>
Link: https://lore.kernel.org/r/20250508184649.2576210-4-jthoughton@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>