Allow instance variables in has filter/order specs

These files create a fake class using an instance variable. While the
proper thing to do would be to refactor the `HasOrders` and `HasFilters`
concerns so they didn't use instance variables but methods, I don't
think that's going to happen in the near future.

.rubocop_basic.yml

@@ -201,6 +201,9 @@ RSpec/HookArgument:
 
 RSpec/InstanceVariable:
   Enabled: true
+  Exclude:
+    - spec/controllers/concerns/has_filters_spec.rb
+    - spec/controllers/concerns/has_orders_spec.rb
 
 RSpec/LetBeforeExamples:
   Enabled: true