* test/etc/test_etc.rb: Etc.getgrnam would not return the first entry in the order of Etc.group for duplicated group names.

follow-up: [Bug #6935]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@60063 b2dd03c8-39d4-4d8f-98ff-823fe69b080e