Move test Node#used? predicate method definition

Definining a method in a `before :all` block doesn't clean it up
automatically, meaning once we've defined it, it's there for all
remaining tests.

Since it leaks anyway, we can just define it at the top of the file, and
avoid needing to disable `ConstantDefinitionInBlock`.

spec/rubocop/ast/node_spec.rb

@@ -2,25 +2,23 @@
 
 require 'uri'
 
+module RuboCop
+  module AST
+    # Patch Node
+    class Node
+      # Let's make our predicate matchers read better
+      def used?
+        value_used?
+      end
+    end
+  end
+end
+
 RSpec.describe RuboCop::AST::Node do
   let(:ast) { parse_source(src).node }
   let(:node) { ast }
 
   describe '#value_used?' do
-    before :all do
-      module RuboCop # rubocop:disable Lint/ConstantDefinitionInBlock
-        module AST
-          # Patch Node
-          class Node
-            # Let's make our predicate matchers read better
-            def used?
-              value_used?
-            end
-          end
-        end
-      end
-    end
-
     context 'at the top level' do
       let(:src) { 'expr' }