iwa: Migrate IWA browser tests to new install flow (from swbn), part 7

Previously, isolated web apps were installed from the DevProxy server;
this method is designed for developers. Final users will install apps
from signed web bundles. The new method emulates that flow by building,
signing, and installing the app.
The migration is divided into several parts due to the large number
of affected files and tests, this CL is the last one.
The remaining two tests using InstallDevModeProxyIsolatedWebApp require
the associated server or dev flow and will therefore not be migrated.

Fixed: 393311329, 361522991
Change-Id: Ib9e7c0b1f3097b934333bea5eea2e7f85993c081
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6218205
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Robbie McElrath <rmcelrath@chromium.org>
Commit-Queue: Robert Ferens <rferens@google.com>
Reviewed-by: Andrew Rayskiy <greengrape@google.com>
Cr-Commit-Position: refs/heads/main@{#1414083}