[PLT-4135] Add lastLoginAt to unit test user fixtures

DbObject requires every declared field in the response payload, so the
issue fixtures need the new field to hydrate a User.

Author: pawelkozik-labelbox

libs/labelbox/tests/unit/schema/test_issue.py

@@ -23,6 +23,7 @@
     "isExternalUser": False,
     "createdAt": _NOW,
     "updatedAt": _NOW,
+    "lastLoginAt": _NOW,
 }
 
 _COMMENT_RAW = {

libs/labelbox/tests/unit/schema/test_project_issues.py

@@ -31,6 +31,7 @@
     "isExternalUser": False,
     "createdAt": _NOW,
     "updatedAt": _NOW,
+    "lastLoginAt": _NOW,
 }
 
 _ISSUE_RAW = {