Improve network callback event tests

We have some tests for the various network callback events. However, there are some gaps in these tests:

- For onBeforeURLRequest, we load a HTML document with 2 subframes - one of these gets redirected in onBeforeURLRequest and the other one gets cancelled. Whilst we check the documentURI for each of the subframes, we don't check we get the appropriate main thread notifications when redirecting or cancelling the request for the main frame (eg, we should verify the sequence of events from WebView.loadEvent). We also don't test that isMainFrame works

- For onBeforeSendHeaders, we load some documents and verify the appropriate header overrides are applied. However, we only do this for the main frame, and don't ever verify isMainFrame. We also don't verify that cancelRequest() works, or verify we get the expected notifications from cancelling the request.

- For onBeforeRedirect, we load some documents and verify that we get the appropriate properties for each notification. In one case, the request is cancelled, but nowhere does the test actually verify that the request was really cancelled - the test doesn't check WebView.loadEvent and doesn't check the actual documentURI, so the test will still pass even if cancelRequest() is a noop