Add a test-only `send_settings` config option

[dongcarl]

1. tests: Remove incorrect driver comments

   In all of these test cases, the driver _is_ polled.

2. tests: Poll driver in header_too_big_discard_from_client

   It is actually safe to poll the driver, since:
   
   1. It only matters that the server has a false impression of the
      client's max_field_section_size and
   2. It doesn't matter if the client knows about the server's
      max_field_section_size or not

3. Add a test-only send_settings config option

   Instead of reading into the server's RequestStream and manually
   overriding its shared state, just have a test-only config option to have
   the client not send its settings.