samples: drivers: i2c: rtio_loopback: add fixture

Add i2c_bus_short fixture to the rtio_loopback sample to ensure it
is only run on boards with the bus shorted. The i2c_bus_short
fixture is also used in the i2c_target_api test suite.

Signed-off-by: Bjarki Arge Andreasen <bjarki.andreasen@nordicsemi.no>