CI bsim workflow: Add UART tests to the bsim workflow

Add new workflow steps to the babblesim workflow to run
also the UART tests on the nrf52_bsim.
This commit:
* Enables the single device tests (which we may
move to the normal twister workflow once we fix the
requirement for a fixture)
* Adds as a placeholder the infra for multidevice tests.

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