Rewrite some of the testOffline.js tests

[marco-c]

Some of the tests in testOffline.js aren't really clever (e.g. they simply test if the name of a file is in the generated service worker code).
We should rewrite them to actually test the service worker code.

[mykmelez]

Esprima could be our friend here.

[marco-c]

We could parse the JS to make them less flaky, or rewrite the tests to run the service worker and check that the behavior is what we expect.

[mykmelez]

We could parse the JS to make them less flaky, or rewrite the tests to run the service worker and check that the behavior is what we expect.

Let's do both!