Unit test XRootD logging has become too verbose

[jhiemstrawisc]

I still think PR #1988, where XRootD log levels were tied to the Logging.Level param, was the right thing to do. However, this means that all our unit tests that spin up a fed-in-a-box are now cranking up XRootD logs, which has resulted in seriously verbose output in GHA -- to the point that it makes you download failed action logs instead of displaying them.

I think the fix should be fairly simple -- pin XRootD log levels to their "error"-level defaults in the fed test instantiation.