Rework EventSource CORS tests to be usable from Workers.

LayoutTests/http/tests/eventsource/script-tests/eventsource-cors-no-server.js

+if (self.importScripts)
+    importScripts("/js-test-resources/js-test.js");
+
+description("Test that EventSource tries to reconnect if there's no server response when making cross-origin requests.");
+
+self.jsTestIsAsync = true;
+
+var count = 0;
+var hosts = ["http://127.0.0.1:12345/event-stream", "http://localhost:54321/event-stream"];
+
+var es;
+
+function runTest() {
+    shouldNotThrow("es = new EventSource(\"" + hosts[count] + "\")");
+    es.onerror = function () {
+        if (es.readyState == es.CONNECTING) {
+            testPassed("got error event and readyState is CONNECTING");
+            es.close();
+        } else
+            shouldBeTrue("es.readyState !== es.CLOSED");
+        if (++count == hosts.length) {
+            finishJSTest();
+            return;
+        }
+        setTimeout(runTest);
+    };
+}
+runTest();