TestDriver: Replace strncpy with strcpy

GCC 8.1.0 now gives an error if `strncpy` is called with a bound
depending on the length of the source argument.  Replace `strncpy` with
`strcpy` as the length is known a priori.

Fixes: #18038

Templates/TestDriver.cxx.in

@@ -48,7 +48,7 @@ static char* lowercase(const char* string)
   if (new_string == NULL) { /* NOLINT */
     return NULL;            /* NOLINT */
   }
-  strncpy(new_string, string, stringSize);
+  strcpy(new_string, string);
   for (p = new_string; *p != 0; ++p) {
     *p = CM_CAST(char, tolower(*p));
   }