tests/websocket: improve portability

This commit improves portability somewhat on machines where sizeof
(int) is less than sizeof (void *). Note that the implementation is
still not portable and will fail when you start using addresses which
can not be represented by "int cast to unsigned int".

On RISC-V 64-bit with RAM on 0x80000000 we previously got hit by
  E: Exception cause Load access fault (5)
    ld      s0,16(a0)
    a0: 0xffffffff80025610

(The int should probably be changed to intptr_t if possible.)

Signed-off-by: Martin Åberg <martin.aberg@gaisler.com>