Add the currently-unused `test_nghttp2_session_create_idle_stream()`
function to the test suite definition.
Modify the test in two places to make it pass:
* Use stream ID=10 as the priority stream ID to test automatic creation
of streams for priority specs. The code below checks against stream
ID=10 so I assume this was a typo in the test.
* Set the `last_sent_stream_id` instead of the `next_stream_id` to test
that idle streams cannot be created with smaller numbers than the
most-recently-seen stream ID. Looking at the validation path in
`session_detect_idle_stream()`, I think this was another test typo.
866eadb5de