bugfix: samples: display: Set ft800 sample to be build only

The sample from samples/drivers/misc/ft800/ causes twister to
timeout as it has no pass conditions defiend nor producess any
output that can be verified. What is more, it requires a shield
which is not reflected in the sample.yaml. This fix sets test harness
to display and adds an entry for the required fixture.

Fixes: #38536

Signed-off-by: Maciej Perkowski <Maciej.Perkowski@nordicsemi.no>