Optimize the `award_user_list` helper spec

According to
https://gitlab.com/gitlab-org/gitlab-ce/issues/23034#note_16586657, each
test for this helper generated 1,833 queries.

Now we only generate stubbed records, and only as many as we need for
each test.

This also corrects a slight logic bug in the helper itself. When the
number of awards was greater than the limit (9 by default), _and_ the
current user was one of them, we actually included 10 names, including
"You", plus the remaining count. Now we return the correct number
regardless.

See merge request !6722