tests: Bluetooth: add pipelined ATT write test

This verifies the zephyr host can tolerate a spec violating peer sending
ATT requests without waiting for responses.

It also tests the opposite, that is that the Zephyr host does not
pipeline requests over the air.

Signed-off-by: Jonathan Rico <jonathan.rico@nordicsemi.no>
Co-authored-by: Aleksander Wasaznik <aleksander.wasaznik@nordicsemi.no>