Add unit tests for the password strength mojo service

Change-Id: I3879b6fc04bd711369a45943d796ea6f285a89fc
Bug: 1345766
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3866197


Commit-Queue: Maria Timbur <mtimbur@google.com>
Reviewed-by: Rafał Godlewski <rgod@google.com>
Reviewed-by: Mohamed Amir Yosef <mamir@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1042056}