Fix http2 auto-test

After it started to fail (somehow it's only OpenSUSE 42.1) again and again
and after a quick re-evaluation it appears the logic testing SETTINGS|ACK
is incorrect. We (client side) start by sending the preface and then
continue to send our request(s). The other side (server) starts from sending its
SETTINGS frame. These settings must be ACKed, but apparently it can happen,
that server receives a requests and sends a reply before it receives SETTINGS|ACK,
resulting in replyFinished (replyFinishedWithError) signal and event loop stopping.
As a result - QVERIFY(serverGotSettingsACK) fails.

Task-number: QTBUG-58758
Change-Id: I8184cf459b2b88f70c646171e0115c184237fad1
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>