samples: shared_mem: fix exception printouts

Enable CONFIG_TEST so that we get the necessary defines for
console output when a fatal error happens, as well as assertion
checking.

Remove an unnecessary self-abort in main(), this causes an
assert to fail. Letting main() return does the same thing, more
gracefully.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>

samples/userspace/shared_mem/prj.conf

@@ -1 +1,2 @@
+CONFIG_TEST=y
 CONFIG_USERSPACE=y

samples/userspace/shared_mem/src/main.c

@@ -161,8 +161,6 @@ void main(void)
 	k_thread_start(&ct_thread);
 	k_sem_give(&allforone);
 	printk("CT thread started\n");
-	k_thread_abort(k_current_get());
-
 }