Run test tearDowns and clean up temporary directories when a signal is caught.

lib/src/runner/browser/iframe_listener.dart

Index: lib/src/runner/browser/iframe_listener.dart
diff --git a/lib/src/runner/browser/iframe_listener.dart b/lib/src/runner/browser/iframe_listener.dart
index 6cef9f62b2022d1a69f25605e3b95a8bb37ca1d7..cdc9edad9321aad065e5434f6b7aee5d99377deb 100644
--- a/lib/src/runner/browser/iframe_listener.dart
+++ b/lib/src/runner/browser/iframe_listener.dart
@@ -143,6 +143,11 @@ class IframeListener {
   void _runTest(Test test, MultiChannel channel) {
     var liveTest = test.load(_suite);
 
+    channel.stream.listen((message) {
+      assert(message['command'] == 'close');
+      liveTest.close();
+    });
+
     liveTest.onStateChange.listen((state) {
       channel.sink.add({
         "type": "state-change",