Currently http2 does not properly submit Goaway frames when
a session is being destroyed. It also doesn't properly
handle when the other party severs the connection after
sending a Goaway frame, even though it should.

It's possible for the connections to take too long and since the
server is already unrefed, the process will just exit. Instead
adjust the test so that server unref only happens after all
session have been successfuly established and unrefed. That still
tests the same condition but will not fail under load.