test: do not fail when parsing PID that isn't thread-group leader (#40677)

test: do not fail when parsing PID that isn't thread-group leader (#40677)

```
TEST-02-UNITTESTS.sh[4382]: [  707.393188] test-cgroup-util[426]: Failed to open pidfd for pid 414: Invalid argument
TEST-02-UNITTESTS.sh[4382]: [  707.393193] test-cgroup-util[426]: src/test/test-cgroup-util.c:249: Assertion failed: Expected "r = proc_dir_read_pidref(d, &pid)" to succeed, but got error: -22/EINVAL
```

The kernel can return EINVAL on pidfd_open() when the selected PID is
not a thread group leader. Don't fail the test, as we are iterating on
everything, so this can seldomly happen.