These accounts are used to verify connection to the physical network/router supplied by BT OpenReach. The reason this works is that BT supply the physical equipment and then the various companies then supply the network out of the exchange. It should be noted that these accounts might not work for all suppliers.

I dont think it matters what the password is except that there is one.

bt_test@startup_domain
adsl

also

bt_test@startup_domain
password

These might also be valid.

bt_test_user@demonhome.dslaccess.co.uk
password

bt_test_user@talktalk.net
password