dnsdist: Fix a memory access violation in the nghttp2 unit tests

dnsdist: Fix a memory access violation in the nghttp2 unit tests

Calling `nghttp2_session_send` from a callback does not work well
when ``nghttp2_session_send`` ends up closing the current stream,
triggering a use-after-free.
It's not clear from the API documentation, but it is mentioned in
the programmers' guide's remarks:

> Do not call `nghttp2_session_send()`, `nghttp2_session_mem_send2()`,
`nghttp2_session_recv()` or `nghttp2_session_mem_recv2()` from the
nghttp2 callback functions directly or indirectly. It will lead to the
crash. You can submit requests or frames in the callbacks then call
these functions outside the callbacks.

Signed-off-by: Remi Gacogne <remi.gacogne@powerdns.com>