tests: thread_runtime_stats: Skip the test on virtual Cortex-R8 platform

Current clocks configuration for the platform makes it impossible
to pass the IDLE_EVENT_STATS_PRECISION check. This is to be addressed
in the future.

Signed-off-by: Mateusz Hołenko <[email protected]>

Author: mateusz-holenko

tests/kernel/usage/thread_runtime_stats/testcase.yaml

@@ -18,3 +18,4 @@ tests:
       - mps2/an385
     platform_exclude:
       - mr_canhubk3
+      - cortex_r8_virtual