tests: drivers: watchdog testing on stm32 iwdg and wwdg

Configure the testcase.yaml to execute watchdog testcase
wdt_basic_api on each IWDG and WWDG on all the stm32 boards
With two generic stm32 .overlay files as extra config :
one wdg is tested when the other is disabled.
Removing from board overlay.
Giving the list of boards else non-stm32 ones could try building

Signed-off-by: Francois Ramu <francois.ramu@st.com>