Add a test runner executable.

lib/src/engine.dart

Index: lib/src/engine.dart
diff --git a/lib/src/engine.dart b/lib/src/engine.dart
index 7bd6958899129492a64c8717905e5ee0d12cde73..5e2b6508c3c450051fa12eb1a5e3fc27f20cb871 100644
--- a/lib/src/engine.dart
+++ b/lib/src/engine.dart
@@ -64,4 +64,9 @@ class Engine {
     }).then((_) =>
         liveTests.every((liveTest) => liveTest.state.result == Result.success));
   }
+
+  /// Signals that the caller is done paying attention to test results and the
+  /// engine should release any resources it has allocated.
+  Future close() =>
+      Future.wait(liveTests.map((liveTest) => liveTest.close()));
 }