[WebAuthn] Refactor layout tests to reduce code.

Instead of custom building each set of options per test,
deep copy the options and remove select properties. Both reduces
code and makes it more clear what is being tested.

Also adds a helper to set up a successful response from the
mock authenticator.

Bug: 664630
Change-Id: If5986e21b56e1093835866283f0e69134f265390
Reviewed-on: https://chromium-review.googlesource.com/957831
Reviewed-by: Balazs Engedy <engedy@chromium.org>
Commit-Queue: Balazs Engedy <engedy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542753}
3 files changed