End-to-end integration test including XRootD

[bbockelm]

Add a go-based integration test that:

1. Launches the origin. May need some modest refactoring to move necessary code from cmd/origin_serve.go into the xrootd or origin_ui module.
2. Checks the xrootd server is alive and ready. Perhaps expose the running PID for xrootd and allow the test to get the resource usage (getrusage - should be exposed in the syscalls package).
3. Checks the health API is set appropriately.
4. Invokes the self-test cycle: upload a file, download a file, delete the file. Verify contents.
5. Shuts down the origin.