Test async channel ID callback.

Start exercising the various async callbacks, starting with channel ID. These
will run under the existing state machine coverage tests; -async will also
enable every asynchronous callback we can.

Change-Id: I173148d93d3a9c575b3abc3e2aceb77968b88f0e
Reviewed-on: https://boringssl-review.googlesource.com/3342
Reviewed-by: Adam Langley <agl@google.com>