h3: fix headers_blocked_on_conn() test
h3: fix headers_blocked_on_conn() test

This test didn't function as fully intended. With this change we ensure
the connection-level flow control gets filled before the request
attempt. Then ensure it is adequately increased by the server draining
some data out of the connection and emitting back MAX_DATA.

cloudflare/quiche,cloudflare/quiche,cloudflare/quiche