tests: make TEST-12-ISSUE-3171 nspawn invocation consistent with other tests

The result of check_nspawn does not mean much, and this forgot to ask
check_nspawn() whether nspawn can be used at all. This brings
TEST-12-ISSUE-3171 in line with other nspawn tests.

test/TEST-12-ISSUE-3171/test.sh

@@ -6,8 +6,12 @@ TEST_DESCRIPTION="https://github.com/systemd/systemd/issues/3171"
 . $TEST_BASE_DIR/test-functions
 
 test_run() {
-    run_nspawn || return 1
-    check_result_nspawn || return 1
+    if check_nspawn; then
+        run_nspawn
+        check_result_nspawn || return 1
+    else
+        dwarn "can't run systemd-nspawn, skipping"
+    fi
     return 0
 }