platform/wmi: string-kunit: Add missing oversized string test case

platform/wmi: string-kunit: Add missing oversized string test case

When compiling the WMI string kunit tests using llvm, the compiler
will issue a warning about "oversized_test_utf8_string" being unused.
This happens because the test case that was supposed to use said
variable was accidentally omitted when adding the kunit tests.

Fix this by adding the aforementioned test case.

Fixes: 0e1a8143e797 ("platform/wmi: Add kunit test for the string conversion code")
Reported-by: Nathan Chancellor <nathan@kernel.org>
Closes: https://lore.kernel.org/platform-driver-x86/20260122234521.GA413183@ax162/
Signed-off-by: Armin Wolf <W_Armin@gmx.de>
Link: https://patch.msgid.link/20260123211537.4448-1-W_Armin@gmx.de
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>