Use strict2 for SelfDrop strict variable specs

Author: karreiro

specs/liquid_ruby/bare_bracket_self.yml

@@ -395,15 +395,15 @@
   template: "{{ self.x }}"
   environment:
     x: 42
-  error_mode: strict
+  error_mode: strict2
   expected: "42"
   hint: |
     Strict variables still allow SelfDrop lookups for keys that are
     defined in the current context.
 
 - name: self_with_strict_variables_returns_empty_for_undefined_var
   template: "{{ self.x }}"
-  error_mode: strict
+  error_mode: strict2
   expected: ""
   hint: |
     Missing keys through SelfDrop return nil and render empty, even