Tests/CustomCommand: Do not use 'main' in a library

Otherwise when linking 'CustomCommand' some linkers report

 ld: 0711-224 WARNING: Duplicate symbol: .main

Suggested-by: Daniel R. Gomez <gomez@teragram.com>

Tests/CustomCommand/GeneratedHeader/main.cpp

@@ -1,5 +1,5 @@
 #include "generated.h"
-int main()
+int mainGeneratedHeader()
 {
   return 0;
 }