ext/curl: update CURLOPT_PREREQFUNCTION test to validate CURLE_ABORTED_BY_CALLBACK return value

Updates the `CURLOPT_PREREQFUNCTION` test to validate that
connections failed when the PREREQFUNC returns abort returns
CURLE_ABORTED_BY_CALLBACK as the error number.

Previously, it only checked against a hardcoded value. Now, it
checks against the `CURLE_ABORTED_BY_CALLBACK` constant as well.

ext/curl/tests/curl_setopt_CURLOPT_PREREQFUNCTION.phpt

@@ -48,6 +48,7 @@ $result = curl_exec($ch);
 var_dump($result);
 var_dump(curl_error($ch));
 var_dump(curl_errno($ch));
+var_dump(curl_errno($ch) === CURLE_ABORTED_BY_CALLBACK);
 
 $returnValue = CURL_PREREQFUNC_OK;
 
@@ -133,6 +134,7 @@ bool(true)
 bool(false)
 string(41) "operation aborted by pre-request callback"
 int(42)
+bool(true)
 
 Testing with CURL_PREREQFUNC_OK
 string(8) "callback"