need better testing around minimum privileges

We've had a number of issues where certain operations checked the wrong privileges and this was not identified by the test suite.

• In #743, I found a bug where we weren't checking a "delete" privilege but the "unauthorized" test succeeded by coincidence because the implementation happened to check the "read" privilege. So the unprivileged user still got a 403/404 (I don't remember which).
• During the lead up to the July demo, we found cases where unprivileged users couldn't see global images or their own ssh keys (#1341). We didn't notice because we'd been doing all our happy-path testing (both automated and otherwise) using "test-privileged", which has all privileges.

The "unauthorized.rs" test verifies that totally unauthenticated or unprivileged users are not able to do things. And the happy path tests verify that superusers are able to do everything. We don't have tests for anything in between. I'll record more thoughts on this below.

Start with the unauthorized.rs tests and compare them with the happy-path tests that use test-privileged. Identify operations covered only for unauthenticated or fully privileged users, then define tests for intermediate privilege levels; done means incorrect privilege checks and access to a user's own or global resources are detected by the suite.