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/shared/system/nested_documentable.rb

@@ -235,12 +235,11 @@ shared_examples "nested documentable" do |login_as_name, documentable_factory_na
       end
       login_as user_to_login
       visit send(path, arguments)
-      FILENAMES ||= %w[clippy empty logo].freeze
 
       send(fill_resource_method_name) if fill_resource_method_name
 
-      documentable.class.max_documents_allowed.times.zip(FILENAMES).each do |_n, fn|
-        documentable_attach_new_file(Rails.root.join("spec/fixtures/files/#{fn}.pdf"))
+      %w[clippy empty logo].take(documentable.class.max_documents_allowed).each do |filename|
+        documentable_attach_new_file(Rails.root.join("spec/fixtures/files/#{filename}.pdf"))
       end
 
       click_on submit_button