Here are the changes needed to achieve this:
* We tag each message posted to the framework with a context argument.
context can be one of "Window", "Worker" or "ServiceWorker". In the
places where we handle messages, we only handle the ones corresponding
to the context that we expect.
* We use Promise.all in order to interleave the execution of the tests.
This has the nice property of actually getting the tests run
simultaneously on multi-core machines.
* For ease of debugging test failures, we print the context from which
each test message is coming from.
* In order for the tests that are run in parallel to not step on each
other's toes, we introduce a global `context' variable that the test
script can use to create names that do not clash with concurrent
test runs. For example, instead of `caches.open("foo")', one must now
use: `caches.open("foo" + context)'.
* The existing tests are modified based on the above.
* When running the tests in parallel, the service worker may end up
controlling both frame.html and message_receiver.html. The hunk at
the end of worker_wrapper.js is intended to ensure that we will
always be communicating with message_receiver.html.
* An order argument has been added to runTests which can be set to
"sequential" in order to run the tests in different contexts in
parallel. If the argument is omitted, the default is "parallel".
