Add autotest for QHttp2ProtocolHandler. This patch contains a very simplistic
"in-process HTTP2 server" for testing the protocol's basic logic/flow control/error
handling and emulating possible scenarios.
Task-number: QTBUG-50956
Change-Id: Ie02d3329c5182277a3c7c84f1bae8d02308e945d
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>