Disallow assertionless tests in Active Model

This commit adds an `after_teardown` logic to the `activemodel` test
suite which ensures that performed test incremented the `assertions`
counter at least once. Otherwise it raises an `AssertionlessTest` error.

This leads to a requirement for tests to be verbose about assertions
even if technically it may not be needed. For example a test like:
```ruby
def test_submitting_a_review_doesnt_raise
  review.submit!
end
```

will have to at least become

```ruby
def test_submitting_a_review_doesnt_raise
  assert_nothing_raised { review.submit! }
end
```

or preferably it should perform a semantically meaningful assertion
that will imply not exception being raised, for example:

```ruby
def test_submitting_a_review_doesnt_raise
  review.submit!

  assert_not_nil review.submitted_at
end
```

Overall while the requirement is being defensive it improves
readability of the tests along with ensuring that we will never
end up having tests that test nothing.

Author: nvasilevski

activemodel/test/cases/attribute_test.rb

@@ -145,10 +145,16 @@ def serialize_cast_value(value)
     end
 
     test "duping does not eagerly type cast if we have not yet type cast" do
-      @type.define_singleton_method(:deserialize) { flunk }
-      attribute = Attribute.from_database(nil, "a value", @type)
+      deserialize_called = false
+      deserialize_called_with = nil
+      @type.define_singleton_method(:deserialize) do |value|
+        deserialize_called_with = value
+        deserialize_called = true
+      end
+      attribute = Attribute.from_database(nil, "my_attribute_value", @type)
 
       attribute.dup
+      assert_not deserialize_called, "deserialize should not have been called, but was called with #{deserialize_called_with}"
     end
 
     class MyType

activemodel/test/cases/helper.rb

@@ -16,6 +16,14 @@
 class ActiveModel::TestCase < ActiveSupport::TestCase
   include ActiveSupport::Testing::MethodCallAssertions
 
+  class AssertionlessTest < StandardError; end
+
+  def after_teardown
+    super
+
+    raise AssertionlessTest, "No assertions made." if passed? && assertions.zero?
+  end
+
   private
     # Skips the current run on JRuby using Minitest::Assertions#skip
     def jruby_skip(message = "")