assert_changes works on including Assertions class

assert_not_equal is an alias for refute_equal that is defined only on
the class ActiveSupport::TestCase. This commit ensures
ActiveSupport::Testing::Assertions#assert_changes doesn't depends on
ActiveSupport::TestCase to work.

activesupport/lib/active_support/testing/assertions.rb

@@ -189,7 +189,7 @@ module ActiveSupport
         error = "#{expression.inspect} didn't change"
         error = "#{error}. It was already #{to}" if before == to
         error = "#{message}.\n#{error}" if message
-        assert_not_equal before, after, error
+        refute_equal before, after, error
 
         unless to == UNTRACKED
           error = "Expected change to #{to}\n"

activesupport/test/testing/method_call_assertions_test.rb

@@ -200,4 +200,20 @@ class MethodCallAssertionsTest < ActiveSupport::TestCase
       assert_equal instance, Level.new
     end
   end
+
+  def test_assert_changes_when_assertions_are_included
+    test_unit_class = Class.new(Minitest::Test) do
+      include ActiveSupport::Testing::Assertions
+
+      def test_assert_changes
+        counter = 1
+        assert_changes(-> { counter }) do
+          counter = 2
+        end
+      end
+    end
+
+    test_results = test_unit_class.new(:test_assert_changes).run
+    assert test_results.passed?
+  end
 end