iwa: Extract MgsMixin from cache test file

It will be helpful for testing IWA Allowlist and Blocklist in the
Managed Guest Session. The class can be also reused in other MGS browser
tests.

Bug: 432229811,307518336
Change-Id: Id17181fc5a22b18d48a5c041eb54615b95604ce8
Fixed: 436856732
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6830444
Reviewed-by: Irina Fedorova <irfedorova@google.com>
Reviewed-by: Sébastien Lalancette <seblalancette@chromium.org>
Commit-Queue: Robert Ferens <rferens@google.com>
Reviewed-by: Andrew Rayskiy <greengrape@google.com>
Reviewed-by: Artem Sumaneev <asumaneev@google.com>
Cr-Commit-Position: refs/heads/main@{#1501423}