tests: kernel: lifo_usage: Exclude on qemu_arc_em

This test has a very high failure rate on `qemu_arc_em` and needs to be
disabled until a fix is available.

To be addressed in #26163.

Signed-off-by: Stephanos Ioannidis <[email protected]>

Author: stephanosio

tests/kernel/lifo/lifo_usage/testcase.yaml

@@ -2,4 +2,4 @@ tests:
   kernel.lifo.usage:
     tags: kernel
     # FIXME: Keeps failing sporadically in CI. See #26163
-    platform_exclude: qemu_arc_hs
+    platform_exclude: qemu_arc_em qemu_arc_hs