Test XML result from computer/api/xml end point

[MarkEWaite]

JENKINS-70074 Test XML result from computer/api/xml end point

An unsuccessful attempt to show the failure reported as JENKINS-70074. It is unclear to me why interactive testing easily duplicated the failure, while this automated test does not show the failure.

Test would not show the expected error on Linux, but did show the expected error on Windows. After applying the fix from #96, the test passed as expected on both Windows and Linux.

I confirmed with JaCoCo coverage reporting that the method modified in #96 is called by this test and was not called by any test before.

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