kconfig: tests: test defconfig when two choices interact

Commit fbe98bb9 ("kconfig: Fix defconfig when one choice menu
selects options that another choice menu depends on") fixed defconfig
when two choices interact (i.e. calculating the visibility of a choice
requires to calculate another choice).

The test code in that commit log was based on the real world example,
and complicated.  So, I shrunk it down to the following:

defconfig.choice:
---8<---
CONFIG_CHOICE_VAL0=y
---8<---

---8<---
config MODULES
        def_bool y
        option modules

choice
        prompt "Choice"

config CHOICE_VAL0
        tristate "Choice 0"

config CHOICE_VAL1
        tristate "Choice 1"

endchoice

choice
        prompt "Another choice"
        depends on CHOICE_VAL0

config DUMMY
        bool "dummy"

endchoice
---8<---

Prior to commit fbe98bb9,

  $ scripts/kconfig/conf --defconfig=defconfig.choice Kconfig.choice

resulted in:

  CONFIG_MODULES=y
  CONFIG_CHOICE_VAL0=m
  # CONFIG_CHOICE_VAL1 is not set
  CONFIG_DUMMY=y

where the expected result would be:

  CONFIG_MODULES=y
  CONFIG_CHOICE_VAL0=y
  # CONFIG_CHOICE_VAL1 is not set
  CONFIG_DUMMY=y

Roughly, this weird behavior happened like this:

Symbols are calculated a couple of times.  First, all symbols are
calculated in conf_read().  The first 'choice' is evaluated to 'y'
due to the SYMBOL_DEF_USER flag, but sym_calc_choice() clears it
unless all of its choice values are explicitly set by the user.

conf_set_all_new_symbols() clears all SYMBOL_VALID flags.  Then, only
choices are calculated.  Here, the SYMBOL_DEF_USER for the first choice
has been forgotten, so it is evaluated to 'm'.  set_all_choice_values()
sets SYMBOL_DEF_USER again to choice symbols.

When calculating the second choice, due to 'depends on CHOICE_VAL0',
it triggers the calculation of CHOICE_VAL0.  As a result, SYMBOL_VALID
is set for CHOICE_VAL0.

Symbols except choices get the final chance of re-calculation in
conf_write().  In a normal case, CHOICE_VAL0 would be re-calculated,
then the first choice would be indirectly re-calculated with the
SYMBOL_DEF_USER which has been recalled by set_all_choice_values(),
which would be evaluated to 'y'.  But, in this case, CHOICE_VAL0 has
already been marked as SYMBOL_VALID, so this re-calculation does not
happen.  Then, =m from the conf_set_all_new_symbols() phase is written
out to the .config file.

Add a unit test for this naive case.

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