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>

tests/qtest/meson.build

@@ -7,6 +7,7 @@ slow_qtests = {
   'npcm7xx_pwm-test': 300,
   'npcm7xx_watchdog_timer-test': 120,
   'qom-test' : 900,
+  'stm32l4x5_usart-test' : 600,
   'test-hmp' : 240,
   'pxe-test': 610,
   'prom-env-test': 360,
@@ -205,7 +206,8 @@ qtests_stm32l4x5 = \
   ['stm32l4x5_exti-test',
    'stm32l4x5_syscfg-test',
    'stm32l4x5_rcc-test',
-   'stm32l4x5_gpio-test']
+   'stm32l4x5_gpio-test',
+   'stm32l4x5_usart-test']
 
 qtests_arm = \
   (config_all_devices.has_key('CONFIG_MPS2') ? ['sse-timer-test'] : []) + \