tst_http2::earlyResponse - fix a flaky and somewhat broken test

1. Fix erroneous logic, which was triggered in 'h2' mode (non-TLS connection)
- after the initial protocol upgrade/POST request was handled, the server
(on Windows specifically) was erroneously handling upcoming DATA frames by replying
with another redirect response.
2. Make the test less heavy by sending 1 MB of Qt::Uninitialize instead of 10 MB
- theoretically this could cause a timeout before the redirected request finished
successfully.

Task-number: QTBUG-73873
Change-Id: I961e0a5f50252988edd46d0e73baf96ee22eef3f
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

tests/auto/network/access/http2/http2srv.cpp

@@ -431,6 +431,13 @@ void Http2Server::readReady()
     if (connectionError)
         return;
 
+    if (redirectSent) {
+        // We are a "single shot" server, working in 'h2' mode,
+        // responding with a redirect code. Don't bother to handle
+        // anything else now.
+        return;
+    }
+
     if (upgradeProtocol) {
         handleProtocolUpgrade();
     } else if (waitingClientPreface) {
@@ -800,6 +807,13 @@ void Http2Server::sendResponse(quint32 streamID, bool emptyBody)
 
     HttpHeader header;
     if (redirectWhileReading) {
+        if (redirectSent) {
+            // This is a "single-shot" server responding with a redirect code.
+            return;
+        }
+
+        redirectSent = true;
+
         qDebug("server received HEADERS frame (followed by DATA frames), redirecting ...");
         Q_ASSERT(targetPort);
         header.push_back({":status", "308"});

tests/auto/network/access/http2/http2srv.h

@@ -193,6 +193,7 @@ private:
     // Redirect, with status code 308, as soon as we've seen headers, while client
     // may still be sending DATA frames.  See tst_Http2::earlyResponse().
     bool redirectWhileReading = false;
+    bool redirectSent = false;
     quint16 targetPort = 0;
     QAtomicInt interrupted;
 protected slots:

tests/auto/network/access/http2/tst_http2.cpp

@@ -473,7 +473,7 @@ void tst_Http2::earlyResponse()
     runEventLoop();
 
     QVERIFY(serverPort);
-    sendRequest(1, QNetworkRequest::NormalPriority, {10000000, Qt::Uninitialized});
+    sendRequest(1, QNetworkRequest::NormalPriority, {1000000, Qt::Uninitialized});
 
     runEventLoop();