[TrustedSignalsFetcher] Disable cookies and add tests.

Also switch network::TestSharedURLLoaderFactory from exposing a
network::NetworkContext to exposing a network::mojom::NetworkContext.
The former is a private services/network class that external consumers
aren't even allowed to depend on, so exposing it in a public test
directory intended to external consumers was not useful. Exposing
the Mojom API gives consumers access to other network APIs, such as
accessing the cookie store in order to inspect on set cookies.

Bug: 333445540
Change-Id: Ife76892137f5604d8b5e22abafee4face3b60482
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5770280
Commit-Queue: mmenke <mmenke@chromium.org>
Reviewed-by: Russ Hamilton <behamilton@google.com>
Cr-Commit-Position: refs/heads/main@{#1340735}