Move custom assertion to its proper place

ActiveSupport::Testing::Assertions.

We have a separate module in which have defined Rails' own custom
assertions. So it would be good to keep all custom Rails' assertions in
one place i.e. in this module.

activesupport/lib/active_support/test_case.rb

@@ -65,16 +65,5 @@ module ActiveSupport
     alias :assert_not_predicate :refute_predicate
     alias :assert_not_respond_to :refute_respond_to
     alias :assert_not_same :refute_same
-
-    # Assertion that the block should not raise an exception.
-    #
-    # Passes if evaluated code in the yielded block raises no exception.
-    #
-    #   assert_nothing_raised do
-    #     perform_service(param: 'no_exception')
-    #   end
-    def assert_nothing_raised
-      yield
-    end
   end
 end

activesupport/lib/active_support/testing/assertions.rb

@@ -19,6 +19,17 @@ module ActiveSupport
         assert !object, message
       end
 
+      # Assertion that the block should not raise an exception.
+      #
+      # Passes if evaluated code in the yielded block raises no exception.
+      #
+      #   assert_nothing_raised do
+      #     perform_service(param: 'no_exception')
+      #   end
+      def assert_nothing_raised
+        yield
+      end
+
       # Test numeric difference between the return value of an expression as a
       # result of what is evaluated in the yielded block.
       #