(PUP-2659) Add a concurrency acceptance test

It is difficult to test concurrent requests with Beaker as it does not
allow parrallel requests to be made. I'm using backgrounded subprocess
in a shell script to get around this, and I'm only making direct catalog
requests via curl, since an agent will halt if it spots another's pidfile in
existence on the same node. Still, this is one of the major calls that
the agent makes, and it does effectively expose PUP-2659 on a
non-synchronized webrick master.