Some tests were failing on darwin,
if the auto-allocate-uids featrure was enabled.
This was because AAU on darwin works by setuid-ing as a non-existent
user, so the tests that were relying on `whoami` were failing.
In the case of trusted-users we fall back to printing the user id,
which is already handled gracefully in the daemon code - i.e. when
a user does not exist or for some other reason looking up their
username is not possible, the daemon falls back to searching for their
uid inside the trusted-users list.
When whoami is used to print the username for other purpose,
we default to printing nixbld.
Change-Id: Ib61615677565098cb5fbf5e26a946ef427c58caf
Nikodem Rabuliński
83a2cd0c46