tests/qtest: Fix STM32L4x5 SYSCFG irq line 15 state assumption

tests/qtest: Fix STM32L4x5 SYSCFG irq line 15 state assumption

The QTest `test_irq_pin_multiplexer` makes the assumption that the
reset state of irq line 15 is low, which is false since STM32L4x5 GPIO
was implemented (the reset state of pin GPIOA15 is high because there's
pull-up and it results in the irq line 15 also being high at reset).

It wasn't triggering an error because `test_interrupt` was mistakenly
"resetting" the line low.

This commit corrects these two mistakes by :
- not setting the line low in `test_interrupt`
- using an irq line in `test_irq_pin_multiplexer` which is low at reset

Signed-off-by: Inès Varhol <ines.varhol@telecom-paris.fr>
Message-id: 20240629104454.366283-1-ines.varhol@telecom-paris.fr
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>