Rework EventSource tests for better Worker test coverage.

LayoutTests/http/tests/eventsource/script-tests/eventsource-retry-precision.js

+if (self.importScripts)
+    importScripts("/js-test-resources/js-test.js");
+
+description("Test EventSource retry time precision.");
+
+self.jsTestIsAsync = true;
+
+var timeoutId = setTimeout(function () {
+    testPassed("did not immediately reconnect");
+    end();
+}, 500);
+
+var count = 0;
+var es = new EventSource("resources/precise-retry.asis");
+es.onopen = function () {
+    if (count++ != 2)
+        return;
+    clearTimeout(timeoutId);
+    testFailed("reconnected too soon");
+    end();
+};
+
+function end() {
+    es.close();
+    debug("DONE");
+    finishJSTest();
+}