Introduce `assert_changes` and `assert_no_changes`

Those are assertions that I really do miss from the standard
`ActiveSupport::TestCase`. Think of those as a more general version of
`assert_difference` and `assert_no_difference` (those can be implemented
by assert_changes, should this change be accepted).

Why do we need those? They are useful when you want to check a
side-effect of an operation. `assert_difference` do cover a really
common case, but we `assert_changes` gives us more control. Having a
global error flag? You can test it easily with `assert_changes`. In
fact, you can be really specific about the initial state and the
terminal one.

```ruby
error = Error.new(:bad)
assert_changes -> { Error.current }, from: nil, to: error do
  expected_bad_operation
end
```

`assert_changes` follows `assert_difference` and a string can be given
for evaluation as well.

```ruby
error = Error.new(:bad)
assert_changes 'Error.current', from: nil, to: error do
  expected_bad_operation
end
```

Check out the test cases if you wanna see more examples.

:beers:

1 files changed, 106 insertions, 0 deletions

diff --git a/activesupport/test/test_case_test.rb b/activesupport/test/test_case_test.rb
index 18228a2ac5..772c3cfca7 100644
--- a/activesupport/test/test_case_test.rb
+++ b/activesupport/test/test_case_test.rb
@@ -111,6 +111,112 @@ class AssertDifferenceTest < ActiveSupport::TestCase
       end
     end
   end
+
+  def test_assert_changes_pass
+    assert_changes '@object.num' do
+      @object.increment
+    end
+  end
+
+  def test_assert_changes_pass_with_lambda
+    assert_changes -> { @object.num } do
+      @object.increment
+    end
+  end
+
+  def test_assert_changes_with_from_option
+    assert_changes '@object.num', from: 0 do
+      @object.increment
+    end
+  end
+
+  def test_assert_changes_with_from_option_with_wrong_value
+    assert_raises Minitest::Assertion do
+      assert_changes '@object.num', from: -1 do
+        @object.increment
+      end
+    end
+  end
+
+  def test_assert_changes_with_to_option
+    assert_changes '@object.num', to: 1 do
+      @object.increment
+    end
+  end
+
+  def test_assert_changes_with_wrong_to_option
+    assert_raises Minitest::Assertion do
+      assert_changes '@object.num', to: 2 do
+        @object.increment
+      end
+    end
+  end
+
+  def test_assert_changes_with_from_option_and_to_option
+    assert_changes '@object.num', from: 0, to: 1 do
+      @object.increment
+    end
+  end
+
+  def test_assert_changes_with_from_and_to_options_and_wrong_to_value
+    assert_raises Minitest::Assertion do
+      assert_changes '@object.num', from: 0, to: 2 do
+        @object.increment
+      end
+    end
+  end
+
+  def test_assert_changes_works_with_any_object
+    retval = silence_warnings do
+      assert_changes :@new_object, from: nil, to: 42 do
+        @new_object = 42
+      end
+    end
+
+    assert_equal 42, retval
+  end
+
+  def test_assert_changes_works_with_nil
+    oldval = @object
+
+    retval = assert_changes :@object, from: oldval, to: nil do
+      @object = nil
+    end
+
+    assert_nil retval
+  end
+
+  def test_assert_changes_with_to_and_case_operator
+    token = nil
+
+    assert_changes 'token', to: /\w{32}/ do
+      token = SecureRandom.hex
+    end
+  end
+
+  def test_assert_changes_with_to_and_from_and_case_operator
+    token = SecureRandom.hex
+
+    assert_changes 'token', from: /\w{32}/, to: /\w{32}/ do
+      token = SecureRandom.hex
+    end
+  end
+
+  def test_assert_no_changes_pass
+    assert_no_changes '@object.num' do
+      # ...
+    end
+  end
+
+  def test_assert_no_changes_with_message
+    error = assert_raises Minitest::Assertion do
+      assert_no_changes '@object.num', '@object.num should not change' do
+        @object.increment
+      end
+    end
+
+    assert_equal "@object.num should not change.\n\"@object.num\" did change to 1.\nExpected: 0\n  Actual: 1", error.message
+  end
 end
 
 class AlsoDoingNothingTest < ActiveSupport::TestCase