test: tst.stackdepth.d marked unstable

The tst.stackdepth.d test is currently unstable because the
sched:::on-cpu probe it depends on requires more work to ensure it
fires correctly in all expected cases.

Signed-off-by: Kris Van Hees <kris.van.hees@oracle.com>
Reviewed-by: Nick Alcock <nick.alcock@oracle.com>

test/unittest/sched/tst.stackdepth.d

@@ -4,6 +4,7 @@
  * Licensed under the Universal Permissive License v 1.0 as shown at
  * http://oss.oracle.com/licenses/upl.
  */
+/* @@tags: unstable */
 
 #pragma D option switchrate=100hz