The test function occasionally can fail on CI/Windows, when a
certificate update on www.qt.io requires installing a new root
certificate on the test VM.
This patch blacklists the test class (which has only one test function)
on Windows. It adds diagnostic output, so future failure reasons can
be established from CI artifacts.
Pick-to: 6.5
Task-number: QTBUG-63481
Change-Id: If686d006f379fe6dbfb0d11d80b8455f72fdbb09
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>