This patch factors out some of the initial test case in
fetch-waits-for-activate.https.html (which only tests "pending" fetch events
for a navigation request) to share with a new test case that tests
"pending" fetch events for a subresource request with a request body.

Both tests in the file have a high-level structure of:
1) Register a Service Worker and wait until its state is "activating" but don't
   let its state reach "activated".
2) Fire a fetch event that will be controlled by that Service Worker, which
   should wait until the Service Worker's state advances to "activated".
3) Wait for the fetch to see that the worker isn't "activated". This step isn't
   directly observable by content, so the test's method to determine this can
   have false posities (but should never cause the test to unexpectedly fail).
4) Tell the Service Worker to advance to "activated".
5) Verify the fetch that was dispatched while the Service Worker was
   "activating" is successfully handled.

Differential Revision: https://phabricator.services.mozilla.com/D49031

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1578919
gecko-commit: 0464faa83b959a5f00f2e6e7951d2818f419c6ac
gecko-integration-branch: autoland
gecko-reviewers: asuth