tests: watchdog: enable wwdg for nucleo_g431rb and nucleo_f091rc

The stm32 wwdg driver was currently tested only on disco_l475_iot1.
Additionally add nucleo_f091rc, such that one board with an CM0 core is
tested. And add nucleo_g431rb, that one board is included that has
newer register mapping and a 128 prescaler.

Signed-off-by: Thomas Stranger <thomas.stranger@outlook.com>