test: fix repeated runs of test-oomd-util by clearing test cgroup

If the test is ran multiple times in a row, without an ephemeral
scope (eg: non-booted nspawn), then subsequent runs will fail as
the test cgroup is not cleared so the previous xattrs are still
present. Trim the test cgroup before and after the test.

Follow-up for 61ff7397d0

src/oom/test-oomd-util.c

@@ -68,6 +68,8 @@ TEST(oomd_cgroup_kill) {
         /* Create another cgroup below this one for the pids we forked off. We need this to be managed
          * by the test so that pid1 doesn't delete it before we can read the xattrs. */
         ASSERT_NOT_NULL(subcgroup = path_join(cgroup, "oomdkilltest"));
+        /* Always start clean, in case of repeated runs and failures */
+        ASSERT_OK(cg_trim(subcgroup, /* delete_root */ true));
         ASSERT_OK(cg_create(subcgroup));
 
         /* If we don't have permissions to set xattrs we're likely in a userns or missing capabilities */
@@ -106,6 +108,8 @@ TEST(oomd_cgroup_kill) {
                 ASSERT_OK(cg_get_xattr(subcgroup, "user.oomd_kill", &v, /* ret_size= */ NULL));
                 ASSERT_STREQ(v, i == 0 ? "2" : "4");
         }
+
+        ASSERT_OK(cg_trim(subcgroup, /* delete_root */ true));
 }
 
 TEST(oomd_cgroup_context_acquire_and_insert) {