kiosk: Introduce FakeOriginTestServerMixin

WebKioskTest used a wide-blanket URLLoaderInterceptor to serve a stable
origin between PRE_ and real tests. In practice this intercepted
unrelated requests to other origins that happened to overlap in timing,
which is undesirable.

EmbeddedTestServer alone is not suitable in this scenario because it
launches with a different port at every run. In web Kiosk a different
port means a different app install URL, which means it becomes a
different Kiosk account altogether. Pinning EmbeddedTestServer to a
fixed port may be problematic if the port is busy.

This CL introduces FakeOriginTestServerMixin to wrap an
EmbeddedTestServer and configure host resolver rules that map a fixed
origin into the possibly variable EmbeddedTestServer address and port.

Fixed: 322497609
Test: testing/xvfb.py tools/autotest.py -C out/Release chrome/browser/ash/login/app_mode/test/web_kiosk_browsertest.cc
Change-Id: I9c03d549d937ae1d6225af29cfbeed9ffc3512c0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5808968
Commit-Queue: Edman Anjos <edman@chromium.org>
Reviewed-by: Jeroen Dhollander <jeroendh@google.com>
Cr-Commit-Position: refs/heads/main@{#1351312}