[XHarness] The parsing of xml results sometimes ignores certain failures.

[mandel-macaque]

The following is an example:

<message>
<![CDATA[ Status not ok Expected: Unauthorized But was: GatewayTimeout ]]>
</message>
<stack-trace>
<![CDATA[ at MonoTests.System.Net.Http.MessageHandlerTest.GHIssue8342 (System.Net.HttpStatusCode expectedStatus, System.String validUsername, System.String validPassword, System.String username, System.String password) [0x000bd] in /Users/builder/jenkins/workspace/xamarin-macios-pr-builder/tests/monotouch-test/System.Net.Http/MessageHandlers.cs:532 ]]>

from #9399

[rolfbjarne]

This happened again here: #9537 (comment)

Test xml (remove .txt extension): vsts-nunit-test-watchos-20200828_092639.xml.txt