tests: Bluetooth: llcp: Add tests to check instant if no PHY change

There were no unit tests that verify if instant value send in
LL_PHY_UPDATE_IND PDU are correct. The commit adds missing tests.

Besides that there is a small update to all ull_cp_phy_update calls,
to improve readability of the code. Magic number was changed to
HOST_INITIATED macro.

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