tests: Align buffer in icmsg_buf tests.
tests: Align buffer in icmsg_buf tests.

This change adds alignment to buffer on which icmsg_buf
is created. Some platforms run into fatal exception
when accessing unaligned addresses. Fix that by ensure
4 byte alignment.

Signed-off-by: Emil Obalski <a38006c9a9911dcd7c7e26537de6a2fa1043ca9e@nordicsemi.no>

zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,finikorg/zephyr