Rework EventSource CORS tests to be usable from Workers.

LayoutTests/http/tests/eventsource/script-tests/eventsource-csp-redirect.js

Index: LayoutTests/http/tests/eventsource/script-tests/eventsource-csp-redirect.js
diff --git a/LayoutTests/http/tests/eventsource/script-tests/eventsource-csp-redirect.js b/LayoutTests/http/tests/eventsource/script-tests/eventsource-csp-redirect.js
new file mode 100644
index 0000000000000000000000000000000000000000..ec01cd0d617468a402add562336cdf8c5321562e
--- /dev/null
+++ b/LayoutTests/http/tests/eventsource/script-tests/eventsource-csp-redirect.js
@@ -0,0 +1,14 @@
+if (self.importScripts)
+    importScripts("/js-test-resources/js-test.js");
+
+description("Test that basic EventSource cross-origin requests fail on blocked CSP redirect.");
+
+self.jsTestIsAsync = true;
+
+var es;
+
+shouldNotThrow("es = new EventSource(\"http://127.0.0.1:8000/resources/redirect.php?code=307&cors_enabled&url=http://127.0.0.1:8080/resources/redirect.php\")");
+es.onerror = function() {
+    shouldBe("es.readyState", "EventSource.CLOSED");
+    finishJSTest();
+};