tests: rename more CURLcode variables to result

For consistency.

Also:
- one remaining in `src/tool_writeout.c`.
- replace casting an `int` to `CURLcode`.
- lib758: rename `CURLMcode` `result` to `mresult`.
- move literals to the right side of if expressions.

Follow-up to d0dc6e2ec0 #20426
Follow-up to 56f600ec23

Closes #20432

tests/libtest/lib1939.c

@@ -35,16 +35,16 @@ static CURLcode test_lib1939(const char *URL)
   if(multi) {
     curl = curl_easy_init();
     if(curl) {
-      CURLcode c;
+      CURLcode result;
       CURLMcode mresult;
 
       /* Crash only happens when using HTTPS */
-      c = curl_easy_setopt(curl, CURLOPT_URL, URL);
-      if(!c)
+      result = curl_easy_setopt(curl, CURLOPT_URL, URL);
+      if(!result)
         /* Any old HTTP tunneling proxy will do here */
-        c = curl_easy_setopt(curl, CURLOPT_PROXY, libtest_arg2);
+        result = curl_easy_setopt(curl, CURLOPT_PROXY, libtest_arg2);
 
-      if(!c) {
+      if(!result) {
 
         /* We are going to drive the transfer using multi interface here,
            because we want to stop during the middle. */