tests/qtest: Check STM32L4x5 clock connections

tests/qtest: Check STM32L4x5 clock connections

For USART, GPIO and SYSCFG devices, check that clock frequency before
and after enabling the peripheral clock in RCC is correct.

Signed-off-by: Inès Varhol <ines.varhol@telecom-paris.fr>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Luc Michel <luc@lmichel.fr>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-id: 20241003081105.40836-4-ines.varhol@telecom-paris.fr
[PMM: Added missing qtest_quit() call]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>