efi_selftest: add hii set keyboard layout test case

Add a test for the case when the HII database protocol
set_keyboard_layout() function is called with a NULL key_guid argument.

Signed-off-by: Vincent Stehlé <vincent.stehle@arm.com>
Cc: Heinrich Schuchardt <xypron.glpk@gmx.de>
Cc: Ilias Apalodimas <ilias.apalodimas@linaro.org>