Add missing Playground tests

tests/examples.rs

@@ -34,3 +34,14 @@ fn trash_compactor() {
         solvers::run_solver(solvers::trash_compactor::TrashCompactor {}, &EXAMPLE_PATHS)
     );
 }
+
+#[test]
+fn playground() {
+    assert_eq!(
+        Ok((40, 0)),
+        solvers::run_solver(
+            solvers::playground::Playground::new(Some(10)),
+            &EXAMPLE_PATHS
+        )
+    );
+}