auth_test: workaround ASan false error
auth_test: workaround ASan false error

test_password_authenticator_operations causes ASan failures, in a way
that I am 99% sure is fully false positive, caused by a combo of
seastar threads, exception throwing and externals.

In lieu of actually identifying what ASan flaw causes this and
potentially cure it, for now, lets just re-write the test in question
to not use seastar::async, but normal continuation. Less easy to read,
but passes ASan.

duarten/scylla,scylladb/scylla,kjniemi/scylla,duarten/scylla,kjniemi/scylla,raphaelsc/scylla,scylladb/scylla,scylladb/scylla,duarten/scylla,avikivity/scylla,scylladb/scylla,kjniemi/scylla,avikivity/scylla,avikivity/scylla,raphaelsc/scylla,raphaelsc/scylla