Bluetooth: Add usage of `util_eq` and `util_memeq`

Update `bt_irk_eq` to use `util_memeq` instead of `memcmp` and the
"disconnect" BabbleSim test to use `util_eq` instead of a first
assertion on the size followed by a `memcmp`.

This is done as an example usage of the two new functions.

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