src/test: avoid (void)! constructs

The reason to use them seems to be that just (void) before a function
call is not enough to silence compiler warnings when return codes are
ignored and -Werror=unused-result is used.

While (void)! apparently works to silence those warnings, it is just too
weird and surprising to readers to use.

It is rather a reason to reconsider the usefulness of the warning.

Closes #22023

src/tool_setopt.c

@@ -226,8 +226,10 @@ static char *c_escape(const char *str, curl_off_t len)
     result = curlx_dyn_addn(&escaped, str, s - str);
 
   if(!result)
-    (void)!curlx_dyn_addn(&escaped, "...", cutoff);
+    result = curlx_dyn_addn(&escaped, "...", cutoff);
 
+  if(result)
+    return NULL;
   return curlx_dyn_ptr(&escaped);
 }