test_struct.rb: use assert_predicate

* test/ruby/test_struct.rb (test_question_mark_in_member): assert_true
  is a method in test-unit. use assert_predicate instead.

* test/ruby/test_struct.rb (test_bang_mark_in_member): ditto.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44045 b2dd03c8-39d4-4d8f-98ff-823fe69b080e