make core_ajax_dart test work in content_shell

content_shell blocks access to external URLs so I just added the response as a file and changed the test so it requests it from `pub serve` instead.


I built a script that runs `pub serve` and then all the tests using `content_shell --dump-render-tree`
Should I create a PR for this script?

When your testing specialists comes up with a better solution we can just delete the script, but I find it convenient to be able to run all tests at once just yet.
Currently all values are hardcoded (path, port, ...) if there is demand I can add command line argument support of course.

I didn't want to introducing an additional dependency. I suppose HOP would offer support for running tests using content_shell, probably others too.