test 'not a CAS' error message as well

tests/heap_lang.ref

@@ -73,3 +73,8 @@
 The command has indeed failed with message:
 Ltac call to "wp_cas_suc" failed.
 Tactic failure: wp_cas_suc: Values are not safe to compare for CAS.
+"not_cas"
+     : string
+The command has indeed failed with message:
+Ltac call to "wp_cas_suc" failed.
+Tactic failure: wp_cas_suc: cannot find 'CAS' in (#())%E.

tests/heap_lang.v

@@ -170,6 +170,13 @@ Section error_tests.
   Proof.
     iIntros "H↦". Fail wp_cas_suc.
   Abort.
+
+  Check "not_cas".
+  Lemma not_cas :
+    (WP #() {{ _, True }})%I.
+  Proof.
+    Fail wp_cas_suc.
+  Abort.
 End error_tests.
 
 Lemma heap_e_adequate σ : adequate NotStuck heap_e σ (= #2).