use method_defined? to check whether or not a method is defined

activesupport/lib/active_support/testing/declarative.rb

@@ -34,7 +34,7 @@ def self.name
         #   end
         def test(name, &block)
           test_name = "test_#{name.gsub(/\s+/,'_')}".to_sym
-          defined = instance_method(test_name) rescue false
+          defined = method_defined? test_name
           raise "#{test_name} is already defined in #{self}" if defined
           if block_given?
             define_method(test_name, &block)