tests: Bluetooth: cte_req: Set missing is_enabled flag

CTE request tests were not working because there was missing
a code that sets conn.llcp.cte_req.is_enabled flag.
If the flag is not set, then the CTE request state machine
comletes its execution immediately as if the request was
disabled by Host in the meantime.

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