tests/lib/time: Fix for posix architecture

* Select always PICOLIBC. This is anyhow the default for all
  but the posix architecture. And for the posix architecture
  we need it as POSIX_CLOCK does not work with the host C
  library.
* Fix the testcase filtering. This sample works fine
  in native_sim (when not using the host libC),
  but does not work for native_posix(_64), as those can
  only be built with the host libC, and in that case,
  the POSIX_CLOCK provided functions will not be properly
  linked to.

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