This switches pub's tests over to using the stream matchers built into
test, as well as the test_descriptor, test_process, and
shelf_test_handler packages. It uses async/await rather than
scheduled_test's task queue.

This removes a layer of indirection which should make it much easier to
work on pub's test infrastructure, especially for people unfamiliar with
scheduled_test.

In particular, "pub get --offline" and "pub cache repair" shouldn't
choke when there are parse failures in the version cache.

Closes #1479