test/vfork00: directly _exit() instead of exec() in vforked proccess

execlp() fails when we run vfork00 test inside namespace because we don't have
'/bin/true' there. Instead of execlp() in vfork-child we can just _exit().

Signed-off-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>