selftests/cgroup: Fix cg_read_strcmp() empty string comparison

cg_read_strcmp() allocated a buffer sized to strlen(expected) + 1,
then passed it to read_text() which calls read(fd, buf, size-1).

When comparing against an empty string (""), strlen("") = 0 gives a
1-byte buffer, and read() is asked to read 0 bytes.  The file content
is never actually read, so strcmp("", buf) always returns 0 regardless
of the real content.  This caused cg_test_proc_killed() to always
report the cgroup as empty immediately, making OOM tests pass without
verifying that processes were killed.

Signed-off-by: Hongfu Li <lihongfu@kylinos.cn>
Signed-off-by: Tejun Heo <tj@kernel.org>