xrootd/client: fix test

Created by: EgorMatirov

Fix a random test failure introduced in #195.

If cancel() is called before client handled request, then the client will raise an error indicating that (expected behaviour if we called Close while some requests were blocked).

In case of test failure, it's safe to call cancel() before the end of request handling because of the fact that the actual error message will still be present at top of the log.