Rework EventSource tests for better Worker test coverage.

LayoutTests/http/tests/eventsource/script-tests/eventsource-events-after-close.js

+if (self.importScripts)
+    importScripts("/js-test-resources/js-test.js");
+
+description("Test that no more message events are fired after EventSource.close() is called, even if it means discarding events that were already processed.");
+
+self.jsTestIsAsync = true;
+var es;
+shouldNotThrow("es = new EventSource(\"/eventsource/resources/event-stream.php\");");
+var counter = 0;
+es.addEventListener('message', function (e) {
+    testPassed("Got message #" + ++counter);
+    if (counter > 1)
+        testFailed("Handler called after the source was closed explicitly.");
+    es.close();
+    // Need to wait to see if we're called again.
+    // event-stream.php sends a bunch of events before flushing, so if close() didn't take
+    // effect we'd get a second message practically instantaneously, waiting 100ms should be ok.
+    setTimeout(finishJSTest, 100);
+}, false);