Duration of tstheap

[PeterBBBBB]

If the duration argument is not defined at start, the test runs forever.
I found for example, running hurd in a VM,
there is no response to CTRL-c,
so the program cannot be halted without killing the VM

if ( argc >= 2 )
   duration = atoi(argv[1]);

Suggest add an else ( duration = 4000 or whatever).