test-cgroup: cleanup test cgroup

One test cgroup gets left behind by the test, as it moves itself
into it. Move itself and back to the original cgroup at the end
and clean up.

This fixes a failure when running the test first as root, and then
as unprivileged (initial cleanup fails as the leftover test cgroup
is owned by root).

src/test/test-cgroup.c

@@ -61,7 +61,7 @@ TEST(cg_create) {
 
         log_info("Paths for test:\n%s\n%s", test_a, test_b);
 
-        /* Possibly clean up left-overs from aboted previous runs */
+        /* Possibly clean up left-overs from aborted previous runs */
         (void) cg_trim(test_a, /* delete_root= */ true);
         (void) cg_trim(test_b, /* delete_root= */ true);
 
@@ -113,6 +113,9 @@ TEST(cg_create) {
 
         ASSERT_OK(cg_trim(test_a, true));
         ASSERT_ERROR(cg_trim(test_b, true), EBUSY);
+
+        ASSERT_OK_ZERO(cg_attach(here, 0));
+        ASSERT_OK(cg_trim(test_b, true));
 }
 
 TEST(id) {