move the Dockerfile to test/apps/echoer

Signed-off-by: Blaž Hrastnik <blaz.hrast@gmail.com>
test: Simplify the test app for TestKey.

Signed-off-by: Blaž Hrastnik <blaz.hrast@gmail.com>
test: Reintroduce waiting for scale events.

That way we ensure the command is working properly internally.

Signed-off-by: Blaž Hrastnik <blaz.hrast@gmail.com>