Fix test leak due to using class instance variables and inheritance

Here's a simple script to illustrate the problem:

```rb
class Foo
  def initialize
    @@options ||= "Foo @@options initialized!"
  end

  def get_options
    @@options
  end
end

class Bar < Foo
  def reset_options
    @@options = "@@options reset in Bar!"
  end
end

Bar.new.reset_options

puts Foo.new.get_options
 #=> "@@options reset in Bar!"
```

Here, the `Configuration` test class defined in
`dynamic_options_test.rb` by inheritance from
`Rails::Railtie::Configuration` ends up overriding the latter's
`@@options` class instance variable.

I'm not sure I understand the general use of `@@` class instance
variables in `Rails::Railstie::Configuration` (I problably wouldn't),
but this fix allows me to remove the leak without changing the
implementation.

To reproduce and confirm the fix, run this test:
```sh-session
$ bin/test test/rails_health_controller_test.rb
test/configuration/dynamic_options_test.rb --seed 15527
```

Author: davidstosik

railties/test/configuration/dynamic_options_test.rb

@@ -7,15 +7,9 @@
 
 module RailtiesTest
   class DynamicOptionsTest < ActiveSupport::TestCase
-    class Configuration < Rails::Railtie::Configuration
-      def reset_options
-        @@options = {}
-      end
-    end
-
     setup do
-      @config = Configuration.new
-      @config.reset_options
+      @config = Rails::Railtie::Configuration.dup.new
+      @config.class.class_variable_set(:@@options, {})
     end
 
     test "arbitrary keys can be set, reset, and read" do