cmCTestScriptHandler: don't call find repeatedly.

Also, prefer the character overload.

Source/CTest/cmCTestScriptHandler.cxx

@@ -330,9 +330,10 @@ int cmCTestScriptHandler::ReadInScript(const std::string& total_script_arg)
   // passed into the scripts as S_ARG
   std::string script = total_script_arg;
   std::string script_arg;
-  if (total_script_arg.find(",") != std::string::npos) {
-    script = total_script_arg.substr(0, total_script_arg.find(","));
-    script_arg = total_script_arg.substr(total_script_arg.find(",") + 1);
+  const std::string::size_type comma_pos = total_script_arg.find(',');
+  if (comma_pos != std::string::npos) {
+    script = total_script_arg.substr(0, comma_pos);
+    script_arg = total_script_arg.substr(comma_pos + 1);
   }
   // make sure the file exists
   if (!cmSystemTools::FileExists(script.c_str())) {