Fix typo in test

A single-char typo in test_auth caused a value that should have
been a simple string into a tuple of strings. This made the test
generate what's actually invalid input data since the username
is supposed to be a simple string.

Change-Id: I7a477bb8e5504c1c7982cf56ffe69e1c5294cadf
Related-bug: #1265108
Merge "Fix typo in test"