kconfig: tests: check visibility of tristate choice values in y choice

If tristate choice values depend on symbols set to 'm', they should be
hidden when the choice containing them is changed from 'm' to 'y'
(i.e. exclusive choice).

This issue was fixed by commit fa64e5f6a35e ("kconfig/symbol.c: handle
choice_values that depend on 'm' symbols").

Add a test case to avoid regression.

For the input in this unit test, there is a room for argument if
"# CONFIG_CHOICE1 is not set" should be written to the .config file.

After commit fa64e5f6a35e, this line was written to the .config file.

With commit cb67ab2cd2b8 ("kconfig: do not write choice values when
their dependency becomes n"), it is not written now.

In this test, "# CONFIG_CHOICE1 is not set" is don't care.

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