tests/qtest: Add tests for the STM32L4x5 USART

Test:
- read/write from/to the usart registers
- send/receive a character/string over the serial port

Signed-off-by: Arnaud Minier <arnaud.minier@telecom-paris.fr>
Signed-off-by: Inès Varhol <ines.varhol@telecom-paris.fr>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20240329174402.60382-6-arnaud.minier@telecom-paris.fr
[PMM: fix checkpatch nits, remove commented out code]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
2 files changed