This adds a `SendRequestExt` trait to h2-support, with a `get` method that does a lot of the repeated request building stuff many test cases were doing. As a first step, the cleans up stream_states tests to use it.
h2 test support
This crate includes utilities for writing h2 tests. This is broken up into a
separate crate because it requires the unstable feature flag and to enable
tower-h2 to use the same helpers.