Should have been removed by 4fd03a4.

It seems to be unused since ddbba26.

Combine TestUserByID and TestUserByName, TestGroupByID and
TestGroupByName, because the test bodies were identical (except for the
called function).

* Remove obsolete tests: They were testing behavior which has changed.
* Fix tests by using either predictable UIDs or replacing UIDs in golden
  files with placeholders (normalizing)
* Limit number of pre-auth user records to avoid denial of service by
  sending many SSH requests for unique users to fill up the UID range.
* Remove field UID from users.UserInfo struct because it's not set in
  internal/brokers/broker.go anymore and there is no need to expose this
  field.
* Rename localgroups package to localentries because it now also
  provides functions for passwd entries.
* Check for error when calling getpwent or getgrent

We only need to unset it once we've used it in a loop, while in the
other cases we'd expect it being set or unset by the function that has
been called

Added tests too