Rework EventSource tests for better Worker test coverage.

LayoutTests/http/tests/eventsource/script-tests/eventsource-parse-event-stream.js

+if (self.importScripts)
+    importScripts("/js-test-resources/js-test.js");
+
+description("Test EventSource text/event-stream parsing.");
+
+self.jsTestIsAsync = true;
+
+var count = -1;
+var es, evt;
+
+shouldNotThrow("es = new EventSource(\"resources/event-stream.php\");");
+es.onopen = function (evt) {
+    testPassed("got open event" + (!evt.data ? "" : " from server"));
+};
+
+es.onmessage = function (arg) {
+    evt = arg;
+    switch(count++) {
+    case -1:
+        shouldBeEqualToString("evt.data", "\n\n");
+        break;
+    case 0:
+        shouldBeEqualToString("evt.data", "simple");
+        break;
+    case 1:
+        shouldBeEqualToString("evt.data", "spanning\nmultiple\n\nlines\n");
+        break;
+    case 2:
+        shouldBeEqualToString("evt.data", "id is 1");
+        shouldBeEqualToString("evt.lastEventId", "1");
+        break
+    case 3:
+        shouldBeEqualToString("evt.data", "id is still 1");
+        shouldBeEqualToString("evt.lastEventId", "1");
+        break;
+    case 4:
+        shouldBeEqualToString("evt.data", "no id");
+        shouldBeEqualToString("evt.lastEventId", "");
+        break;
+    case 5:
+        shouldBeEqualToString("evt.data", "a message event with the name \"message\"");
+        break;
+    case 6:
+        shouldBeEqualToString("evt.data", "a line ending with crlf\na line with a : (colon)\na line ending with cr");
+        break;
+    default:
+        testFailed("got unexpected event");
+        es.close();
+    }
+};
+
+es.onerror = function () {
+    es.close();
+
+    shouldBe("count", "7");
+    debug("DONE");
+    finishJSTest();
+};