Rework EventSource CORS tests to be usable from Workers.

LayoutTests/http/tests/eventsource/workers/eventsource-cors-with-credentials-expected.txt

+CONSOLE ERROR: EventSource cannot load http://127.0.0.1:8080/eventsource/resources/es-cors-credentials.php?count=1. A wildcard '*' cannot be used in the 'Access-Control-Allow-Origin' header when the credentials flag is true. Origin 'http://127.0.0.1:8000' is therefore not allowed access.
+CONSOLE ERROR: EventSource cannot load http://127.0.0.1:8080/eventsource/resources/es-cors-credentials.php?count=2. A wildcard '*' cannot be used in the 'Access-Control-Allow-Origin' header when the credentials flag is true. Origin 'http://127.0.0.1:8000' is therefore not allowed access.
+CONSOLE ERROR: EventSource cannot load http://127.0.0.1:8080/eventsource/resources/es-cors-credentials.php?count=3. Credentials flag is 'true', but the 'Access-Control-Allow-Credentials' header is ''. It must be 'true' to allow credentials.
+[Worker] Test that EventSource cross-origin requests with credentials fail until the correct CORS headers are sent.
+
+On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE".
+
+
+Starting worker: ../script-tests/eventsource-cors-with-credentials.js
+PASS [Worker] es = new EventSource("http://127.0.0.1:8080/eventsource/resources/es-cors-credentials.php?count=1", {'withCredentials': true}) did not throw exception.
+PASS [Worker] es.withCredentials is true
+PASS [Worker] count != 4 && count != 5 is true
+PASS [Worker] es = new EventSource("http://127.0.0.1:8080/eventsource/resources/es-cors-credentials.php?count=2", {'withCredentials': true}) did not throw exception.
+PASS [Worker] es.withCredentials is true
+PASS [Worker] count != 4 && count != 5 is true
+PASS [Worker] es = new EventSource("http://127.0.0.1:8080/eventsource/resources/es-cors-credentials.php?count=3", {'withCredentials': true}) did not throw exception.
+PASS [Worker] es.withCredentials is true
+PASS [Worker] count != 4 && count != 5 is true
+PASS [Worker] es = new EventSource("http://127.0.0.1:8080/eventsource/resources/es-cors-credentials.php?count=4", {'withCredentials': true}) did not throw exception.
+PASS [Worker] es.withCredentials is true
+PASS [Worker] evt.origin.indexOf('http://127.0.0.1:8080') === 0 is true
+PASS [Worker] evt.data is "DATA1"
+PASS [Worker] evt.lastEventId is "77"
+PASS [Worker] count is 5
+PASS [Worker] evt.origin.indexOf('http://127.0.0.1:8080') === 0 is true
+PASS [Worker] count is 5
+PASS [Worker] evt.data is "DATA2"
+PASS successfullyParsed is true
+
+TEST COMPLETE
+