tests/bsim: Also runtime test the UART drivers for the nrf54l15

To increase coverage.

Note that we call twister separatedly for this target, due to the 54l15
overlays using UARTE20 which is indexed in simulation as "2", but as
the nrf52833 does not have 3 instances. So "-uart2_loopback" is not
a valid option for a 52833 executable.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>