tests for running as a normal user

[sbrudenell]

I tried adding some naive tests to run as a normal user, but among other things, btrfs send fails with EPERM-related failures.

the send ioctl, and others, require CAP_SYS_ADMIN. in order to simulate a non-root user with this, we need to retain the capability while dropping root. the dance to do this is documented here. but the linux capability-related functions aren't currently mapped in python (an ancient bug).

I expect the best move is to call these functions with ctypes.