sys: timeutil & test: Multiple fixes

* Fixes in code and test so it works with any expected time_t size.
* Fix test_timespec_normalize so it also tests the output of
  timespec_normalize(), fixing some incorrect expected values
  accordingly.
* Test: Do not force a C library in the integration platform
  and do not filter by libC as it is not necessary anymore.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>