Remove instance variables in RSpec

Instance variables might lead to hard-to-detect issues, since using a
nonexistent instance variable will return `nil` instead of raising an
error.

.rubocop_basic.yml

@@ -199,6 +199,9 @@ RSpec/Focus:
 RSpec/HookArgument:
   Enabled: true
 
+RSpec/InstanceVariable:
+  Enabled: true
+
 RSpec/LetBeforeExamples:
   Enabled: true