platform/wmi: Add kunit test for the string conversion code

platform/wmi: Add kunit test for the string conversion code

The string conversion frunctions provided by the WMI driver core
have no dependencies on the remaining WMI API, making them suitable
for unit tests.

Implement such a unit test using kunit. Those unit tests verify that
converting between WMI strings and UTF8 strings works as expected.
They also verify that edge cases are handled correctly.

Signed-off-by: Armin Wolf <W_Armin@gmx.de>
Link: https://patch.msgid.link/20260116204116.4030-5-W_Armin@gmx.de
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>