tests: Added test for UTF-8 characters in Kconfig values

Added a test for having UTF-8 characters in Kconfig values. This
ensures that issue
https://github.com/zephyrproject-rtos/zephyr/issues/6649

 does not
affect any supported platforms and that it does not re-appear in the
future.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>