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>

tests/lib/time/prj.conf

@@ -1,2 +1,3 @@
 CONFIG_ZTEST=y
 CONFIG_POSIX_CLOCK=y
+CONFIG_PICOLIBC=y

tests/lib/time/testcase.yaml

@@ -1,7 +1,9 @@
 tests:
   libraries.libc.time:
     tags: libc
-    filter: not CONFIG_ARCH_POSIX or CONFIG_NATIVE_POSIX_SLOWDOWN_TO_REAL_TIME
     timeout: 180
     integration_platforms:
       - mps2_an385
+    platform_exclude:
+      - native_posix
+      - native_posix_64