llext: hotfix: fix device API tests for platforms without console

LLEXT tests currently fail on platforms where a console was chosen but
its underlying device is not enabled. This might happen for example on
multicore SoCs, where the console is only enabled on the most powerful
CPU.

Fix this by only allowing testing the device API if the console device
is both chosen and enabled in the final DT.

Signed-off-by: Luca Burelli <l.burelli@arduino.cc>