Bluetooth: testlib: Create `testlib/conn.h`

This header file declares the functions that rely on `bluetooth/conn.h`.

This collects `testlib/connect.h`, `conn_ref.h` and `conn_wait.h` into one.

Initial documentation for the functions in this file is also added.

Signed-off-by: Aleksander Wasaznik <aleksander.wasaznik@nordicsemi.no>