Using a smaller viewport speeds up both tests, and lets us know what
dimensions to expect in TestCaptureScreenshot.
For TestScreenshot, we can know what dimensions to expect in advance, as
we have the images in testdata.
'go test -run Screenshot' goes from ~0.9s to ~0.5s on my machine.
Finally, don't run ExampleTitle as part of 'go test', as it's slow.
The Default folder is created asynchronously to Chrome starting to
listen on the debugging protocol port. So we can't expect it to exist.
Instead, base the example on DevToolsActivePort, which we can rely on.
All it did was wait on the entire allocator, which is confusing. From
the user's perspective, this wait method should instead wait for the
resources for its own browser, and not any other browsers sharing the
same allocator.
We haven't decided how to integrate that into our API, so simply replace
it with Allocator.Wait.
