lib: kunit: add bitfield test conversion to KUnit

This adds the conversion of the runtime tests of test_bitfield,
from `lib/test_bitfield.c` to KUnit tests.

Code Style Documentation: [0]

Signed-off-by: Vitor Massaru Iha <vitor@massaru.org>
Link: [0] https://lore.kernel.org/linux-kselftest/20200620054944.167330-1-davidgow@google.com/T/#u


Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>