[tests] fixes obsolete tests that asked for long-gone msg keys

src/plugins/wp/tests/wp_hoare/dispatch_var2.i

 /* run.config
-   OPT: -wp-model +ref -wp-no-let -wp-msg-key var_kind
-   OPT: -wp-model +ref -wp-let    -wp-msg-key var_kind
+   OPT: -wp-model +ref -wp-no-let
+   OPT: -wp-model +ref -wp-let
 */
 
 /* run.config_qualif

src/plugins/wp/tests/wp_hoare/logicref.i

 /* run.config
-   OPT: -wp-model +ref -wp-msg-key var_kind
+   OPT: -wp-model +ref
 */
 
 /* run.config_qualif

src/plugins/wp/tests/wp_hoare/reference.i

 /* run.config
-   OPT: -wp-model +ref -wp-msg-key var_kind
+   OPT: -wp-model +ref
 */
 
 /* run.config_qualif

src/plugins/wp/tests/wp_hoare/reference_and_struct.i

 /* run.config
-   OPT: -wp-model +ref -wp-msg-key var_kind
+   OPT: -wp-model +ref
 */
 
 /* run.config_qualif

src/plugins/wp/tests/wp_hoare/reference_array.i

 /* run.config
-   OPT: -wp-model +ref -wp-msg-key var_kind
+   OPT: -wp-model +ref
 */
 
 /* run.config_qualif

tests/impact/loop2.i

 /* run.config
-   STDOPT: #"-kernel-msg-key offsetmap:caches -impact-pragma main -lib-entry -calldeps" +"-then -ulevel 10"
+   STDOPT: #"-impact-pragma main -lib-entry -calldeps" +"-then -ulevel 10"
    */
 
 

tests/misc/plugin_log.ml

 open Kernel
 
+let dkey = register_category "foo-category"
+
 let main () =
-  let dkey = register_category "foo-category" in
   result ~dkey "result with dkey";
   result "result";
   feedback ~dkey "feedback with dkey";