Fix test class name for `Assertions` module

Because this class includes not only `assert_difference` but also tests
of other assertion methods.

activesupport/test/test_case_test.rb

@@ -2,7 +2,7 @@
 
 require "abstract_unit"
 
-class AssertDifferenceTest < ActiveSupport::TestCase
+class AssertionsTest < ActiveSupport::TestCase
   def setup
     @object = Class.new do
       attr_accessor :num