[autotest] Add unqiue constraint on afe_users.login

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

BUG=chromium:395262
TEST=ran migrate
DEPLOY=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>
1 file changed