coin agent should not fail the integration when "error unmarshalling testlib xml output" happens

In QTBUG-80527, the QML tests write invalid UTF-8 characters in the XML output. Even though the build is green and all tests pass, coin-agent fails unmarshalling the XML, and the integration fails. If you search for this error message, it has happened a few times in the past.

This ticket is about making the Coin agent more resilient, not failing a whole integration because of a couple of invalid characters.

• it could replace invalid utf-8 with '?'
• the build should pass, since all tests are green anyway. It's the coin agent that is failing here, not any of the tests
• it could print the error anyway with a message like "this is a problem, please report it". Gerrit bot could even pass this message to the codereview page.