tests: drivers: uart: uart_pm: Disable PM_DEVICE_SYSTEM_MANAGED

When PM_DEVICE_SYSTEM_MANAGED is enabled (and that's the default) all
devices are suspended when device go to idle state (through pm
subsystem and not through just cpu_idle). In this test we are manually
controlling PM of the DUT so we don't want that.

Additionally, suspend action is taking a semaphore and it is illegal
in idle thread (where system managment operates) which causes assert
in kernel scheduler.

Removed for nrf54h20 as it applies only there.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>