Bluetooth: tests: Switch to default `BT_RECV_CONTEXT`

The tests `hci_prop_evt` and `host_long_adv_recv` used `bt_recv_prio`,
and so relied on an extra config option `CONFIG_BT_RECV_BLOCKING`. This
patch replaces the use of `bt_recv_prio` with `bt_recv` and restores the
use of the default option for `BT_RECV_CONTEXT`.

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