test,systemctl,nspawn: use "const char*" instead of "char*" as iterator for FOREACH_STRING()

The macro iterates through literal strings (i.e. constant strings),
hence it's more correct to have the iterator const too.

src/test/test-execute.c

@@ -131,7 +131,7 @@ static bool check_user_has_group_with_same_name(const char *name) {
 }
 
 static bool is_inaccessible_available(void) {
-        char *p;
+        const char *p;
 
         FOREACH_STRING(p,
                 "/run/systemd/inaccessible/reg",