Tests: Bluetooth: Fix conn_stress compilation warn

On newer GCC version warnings were emitted because of a `memcpy`. The
size parameter was larger than the source's size because it was using
the size of the destination, which was a `bt_uuid_128` when the source
is a `bt_uuid_16`.

Fix the issue by creating a new variable for CCC UUID in both the
central and peripheral code. The variable need to be static because the
discover parameters must remain valid until the start of the discover
attribute callback.

Signed-off-by: Théo Battrel <theo.battrel@nordicsemi.no>