tests: bsim: bluetooth: Remove bt_hci_cmd_create() usage

These tests have some sort of simple host implementation, which reuses the
bt_hci_cmd_create() API name, but the implementation is actually local and
something that can be made static. Change the function to be static and
rename it to be more in line with other internal functions.

Signed-off-by: Johan Hedberg <johan.hedberg@silabs.com>