Ensure user removal includes from any groups.

Otherwise, we end up with colleges which claim to have a non-existent
user as a member, which then throws all sorts of exceptions.

Change-Id: I4557928f90b7ea3a18405d06e53f5d74173219d8
Have these tests use a unique username.

This is easier to pick up on when things go wrong.

Change-Id: I7fd54f77c55b661aff1948c1e570a094d11417ef
Actually make the user a student when they activate.

Without this, they probably wouldn't have been able to do much.

Change-Id: I5b0b9993d2bdeb8ab0acaa9bfa6fbe54b8891e84