Add and apply ConstantDefinitionInBlock rule

This rule was added in Rubocop 0.91.0. A similar rule named
LeakyConstantDeclaration was added in rubocop-rspec 1.34.0.

Note using the FILENAMES constant did not result in an offense using the
ConstantDefinitionInBlock rule but did result in an offense using the
LeakyConstantDeclaration rule. I've simplified the code to get rid of
the constant; not sure why we were adding a constant with `||=` in the
middle of a spec.

spec/lib/consul_form_builder_spec.rb

@@ -1,10 +1,14 @@
 require "rails_helper"
 
 describe ConsulFormBuilder do
-  class DummyModel
-    include ActiveModel::Model
-    OPTIONS = %w[Good Bad Ugly].freeze
-    attr_accessor :title, :quality
+  before do
+    dummy_model = Class.new do
+      include ActiveModel::Model
+      attr_accessor :title, :quality
+    end
+
+    stub_const("DummyModel", dummy_model)
+    stub_const("DummyModel::OPTIONS", %w[Good Bad Ugly].freeze)
   end
 
   let(:builder) { ConsulFormBuilder.new(:dummy, DummyModel.new, ActionView::Base.new, {}) }