alternator-test: add a wrong password test
alternator-test: add a wrong password test

The additional test case submits a request as a user that is expected
to exist (in the local setup), but the provided password is incorrect.
It also updates test_wrong_key_access so it uses an empty string
for trying to authenticate as an inexistent user - in order to cover
more corner cases.

avikivity/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla