Make SignalR C++ tests output test results to AzP

[analogrelay]

GTest supports outputting a junit-style XML file: https://github.com/google/googletest/blob/master/googletest/docs/advanced.md#generating-an-xml-report

We should do that and then make sure AzP is scooping up junit xml files in a Publish Test Results task.

Coordinate this work with #8724 and #8438 so that we don't have like 4 AzP tasks scooping XML files :). We should publish to a common directory (artifacts/logs/junit?) and a single publishing task scooping from that directory.