tests: mbox_api: a bit more stack for qemu_x86_tiny

For testing on qemu_x86_tiny, a little bit more stack is needed.
So add the extra stack for testing.

Signed-off-by: Daniel Leung <[email protected]>

Author: dcpleung

tests/kernel/mbox/mbox_api/boards/qemu_x86_tiny.conf

@@ -0,0 +1,5 @@
+# Copyright (c) 2021 Intel Corporation
+# SPDX-License-Identifier: Apache-2.0
+
+# Need a little bit more stack space
+CONFIG_TEST_EXTRA_STACKSIZE=256