add missing fixture to test/unit/issue_relation_test.rb

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@12069 e93f8b46-1217-0410-a6f0-8f06a7374b81
not use assert_not_nil in Errors#[]

r7593 etc. replaced Rails2 Errors#on.
Rails3 Errors#[] always return array.
So, Rails3 Errors#[] is always not nil.

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@12070 e93f8b46-1217-0410-a6f0-8f06a7374b81