kconfig: tests: add basic choice tests

The calculation of 'choice' is a bit complicated part in Kconfig.

The behavior of 'y' choice is intuitive.  If choice values are tristate,
the choice can be 'm' where each value can be enabled independently.
Also, if a choice is marked as 'optional', the whole choice can be
invisible.

Test basic functionality of choice.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Ulf Magnusson <ulfalizer@gmail.com>