tests: Bluetooth: bsim_advx_test: Add some tests for scan response

This adds few tests for handling scan response in ext commands:
- trying to set scan response on non-scannable instance should fail
- it should be possible to create scannable instance, set scan data,
  enable and then disable set

Signed-off-by: Andrzej Kaczmarek <andrzej.kaczmarek@codecoup.pl>