tests: Bluetooth: df: Wrong feature set for features_used

Wrong feature was enabled for a connection object as one the currently
used features. The enabled feature was CTE response. In case if the
feature exchange has not happened, it is verified if CTE request feature
is enabled, not the CTE response feature. The problem is in a common
code that prepares connection object for unit tests.

The commit fixes enabled features preparation for a connection object.

Signed-off-by: Piotr Pryga <piotr.pryga@nordicsemi.no>