[autotest] Add unqiue constraint on afe_users.login

Add unique constraint to avoid duplicate login names, which happened on moblab.

TEST=ran migrate

Change-Id: Idbf08957a0b181013e4bb7951c002cd078bcc44e
Reviewed-on: https://chromium-review.googlesource.com/209167
Tested-by: Jiaxi Luo <jiaxiluo@chromium.org>
Reviewed-by: Simran Basi <sbasi@chromium.org>
Reviewed-by: Alex Miller <milleral@chromium.org>
Commit-Queue: Jiaxi Luo <jiaxiluo@chromium.org>
diff --git a/frontend/migrations/093_make_afe_users_login_unique.py b/frontend/migrations/093_make_afe_users_login_unique.py
new file mode 100644
index 0000000..4b37f16
--- /dev/null
+++ b/frontend/migrations/093_make_afe_users_login_unique.py
@@ -0,0 +1,7 @@
+UP_SQL = """
+CREATE UNIQUE INDEX login_unique ON afe_users (login);
+DOWN_SQL = """
+DROP INDEX login_unique ON afe_users;
\ No newline at end of file