test_assignment.rb: split test_assign

* test/ruby/test_assignment.rb (test_assign): split by assignment
  forms.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46363 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
test_assignment.rb: assignment to private attribute

* test/ruby/test_assignment.rb (test_assign_private_self): test
  for r3509, assignment to private attribute is allowed iff its
  receiver is literal `self`.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46364 b2dd03c8-39d4-4d8f-98ff-823fe69b080e