Add system tests for admin commands

Problem to solve

• Currently there is no comprehensive test coverage for admin commands, so we cannot detect regressions.
• We also cannot verify that output from the XRootD SSI Frontend and the gRPC Frontend is the same.

Stakeholders

• CTA developers
• CTA operators
• External sites

Proposal

• Add a system test which systematically exercises the output from every single CTA admin command.