Fix gutils-user-database unit test

The gutils-user-database.c test is broken in two ways and currently doesn't test anything:

• It only overrides getpwuid, where the implementation uses getpwnam_r (if LOGNAME is set and correct) or getpwuid_r (otherwise) if they exist, which should be every system for at least 20 years.
• It only partly cargo-culted setting the environment for the local and installed tests, but failed to actually set the environment for either.