Disable cipherstring-setting test for schannel

And put a note about it in the documentation

Change-Id: I29126e4a80f83c256190e03b8fe01f3c869fd46d
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>

src/network/ssl/qsslconfiguration.cpp

@@ -624,6 +624,8 @@ void QSslConfiguration::setCiphers(const QList<QSslCipher> &ciphers)
     must be done before the handshake phase, where the session cipher
     is chosen.
 
+    \note This is not currently supported in the Schannel backend.
+
     \sa ciphers()
 */
 void QSslConfiguration::setCiphers(const QString &ciphers)

tests/auto/network/ssl/qsslsocket/tst_qsslsocket.cpp

@@ -822,9 +822,13 @@ void tst_QSslSocket::ciphers()
     if (!ciphers.size())
         QSKIP("No proper ciphersuite was found to test 'setCiphers'");
 
+#if QT_CONFIG(schannel)
+    qWarning("Schannel doesn't support setting ciphers from a cipher-string.");
+#else
     sslConfig.setCiphers(ciphersAsString);
     socket.setSslConfiguration(sslConfig);
     QCOMPARE(ciphers, socket.sslConfiguration().ciphers());
+#endif
     sslConfig.setCiphers(ciphers);
     socket.setSslConfiguration(sslConfig);
     QCOMPARE(ciphers, socket.sslConfiguration().ciphers());