unicode: kunit: refactor selftest to kunit tests

unicode: kunit: refactor selftest to kunit tests

Refactoring 'test' functions into kunit tests, to test utf-8 support in
unicode subsystem.

This allows the utf8 tests to be run alongside the KUnit test suite
using kunit-tool, quickly compiling and running all desired tests as
part of the KUnit test suite, instead of compiling the selftest module
and loading it.

The refactoring kept the original testing logic intact, while adopting a
testing pattern across different kernel modules and leveraging KUnit's
benefits.

Co-developed-by: Pedro Orlando <porlando@lkcamp.dev>
Signed-off-by: Pedro Orlando <porlando@lkcamp.dev>
Co-developed-by: Danilo Pereira <dpereira@lkcamp.dev>
Signed-off-by: Danilo Pereira <dpereira@lkcamp.dev>
Signed-off-by: Gabriela Bittencourt <gbittencourt@lkcamp.dev>
Reviewed-by: David Gow <davidgow@google.com>
Acked-by: Gabriel Krisman Bertazi <krisman@suse.de>
Reviewed-by: Rae Moar <rmoar@google.com>
Link: https://lore.kernel.org/r/20241202075545.3648096-6-davidgow@google.com
Signed-off-by: Kees Cook <kees@kernel.org>