tst_http2::goaway - run in ALPN mode only

I was observing rare crashes on my mac, where I was using SecureTransport.
This would imply both the client (the test) and the server were working
in h2c mode. But this is against the test's logic - the first request
will be HTTP/1.1 (and upgrading protocol) and I wanted to send 3 HTTP/2
requests instead.

Fixes: QTBUG-77476
Change-Id: I048ca242e2096ca36dd112277807d1fee530150c
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

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

@@ -455,6 +455,9 @@ void tst_Http2::goaway_data()
     // - server waits for some time (enough for ur to init several streams on a
     // client side); then suddenly it replies with GOAWAY, never processing any
     // request.
+    if (clearTextHTTP2)
+        QSKIP("This test requires TLS with ALPN to work");
+
     QTest::addColumn<int>("responseTimeoutMS");
     QTest::newRow("ImmediateGOAWAY") << 0;
     QTest::newRow("DelayedGOAWAY") << 1000;