QDtls(cookie verifier): make sure a server can re-use 'Client Hello'

And extend an auto-test for this. When a cookie verification mechanism is
enabled, and verifier, indeed, verifies that some datagram is a 'Client Hello'
message with a proper cookie attached, we start a real DTLS handshake creating a
QDtls object and calling 'doHandshake'. In case cookie verification
was enabled, we need parameters from the verifier (it's a crypto-strong
'number' and hash algorithm) to 'lock and load' the TLS state machine in
a freshly created TLS session object. This code path previously was only
tested manually and was found by LCOV as untested.

Pick-to: 5.15
Pick-to: 6.0
Change-Id: Ieacb8c989997999ea10e15bda6ae106a0338b698
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

tests/auto/network/ssl/qdtlscookie/tst_qdtlscookie.cpp

@@ -288,6 +288,20 @@ void tst_QDtlsCookie::verifyClient()
                                           clientPort), true);
     QCOMPARE(anotherListener.verifiedHello(), dgram);
     QCOMPARE(anotherListener.dtlsError(), QDtlsError::NoError);
+
+    // Now, let's test if a DTLS server is able to create a new TLS session
+    // re-using the client's 'Hello' with a cookie inside:
+    QDtls session(QSslSocket::SslServerMode);
+    auto dtlsConf = QSslConfiguration::defaultDtlsConfiguration();
+    dtlsConf.setDtlsCookieVerificationEnabled(true);
+    session.setDtlsConfiguration(dtlsConf);
+    session.setPeer(clientAddress, clientPort);
+    // Borrow a secret and hash algorithm:
+    session.setCookieGeneratorParameters(listener.cookieGeneratorParameters());
+    // Trigger TLS state machine change to think it accepted a cookie and started
+    // a handshake:
+    QVERIFY(session.doHandshake(&serverSocket, dgram));
+
     // Now let's use a wrong port:
     QCOMPARE(listener.verifyClient(&serverSocket, dgram, clientAddress, serverPort), false);
     // Invalid cookie, no verified hello message: