kconfig: Fix BrokenPipeError warnings in selftests

kconfig: Fix BrokenPipeError warnings in selftests

The kconfig test harness ("make testconfig") was generating BrokenPipeError
warnings when running interactive tests like oldaskconfig and oldconfig:

  /usr/lib/python3/dist-packages/_pytest/unraisableexception.py:85: PytestUnraisableExceptionWarning: Exception ignored in: <_io.BufferedWriter name=12>

  Traceback (most recent call last):
    File "/srv/code/scripts/kconfig/tests/conftest.py", line 127, in oldaskconfig
      return self._run_conf('--oldaskconfig', dot_config=dot_config,
             ~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
                            interactive=True, in_keys=in_keys)
                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  BrokenPipeError: [Errno 32] Broken pipe

The issue occurred when the test framework attempted to write to stdin
after the conf subprocess had already exited.

Wrap stdin write operations in try/except to catch BrokenPipeError and
stop sending more input. Add explicit flush() after writes so we can see
delivery errors immediately. Ignore BrokenPipeError when closing stdin.
Explicitly call wait() to validate subprocess termination.

Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Link: https://lore.kernel.org/r/20250923213422.1105654-1-kees@kernel.org
Signed-off-by: Kees Cook <kees@kernel.org>