tests: fill old_value argument in timer{,fd}_xettime tests

tests: fill old_value argument in timer{,fd}_xettime tests

This is needed in order to differentiate it from the value returned
by the call.  As a consequence, it enables revealing possible bugs
in syscall parsers, for example, when the value read on syscall
entering and not on syscall exiting, as it was the case with
timerfd_settime parser.

* tests/timer_xettime.c (main): Fill old.its field with value different
from the expected one upon call return.
* tests/timerfd_xettime.c: Likewise.