Fix: BtmPageVisitObserver reported client redirects as server redirects

Before this CL, BtmPageVisitReporter was populating
BtmNavigationInfo.server_redirects with all the redirects from
NavigationHandle::GetRedirectChain(). However, true client redirects
(such as via an HTML <meta> tag) are also returned by
GetRedirectChain().

This CL also extracts some helpers for client-side redirect
browsertesting from btm_navigation_flow_detector_browsertest.cc to
btm_test_utils.{h,cc}, to assist with new browsertests in
btm_page_visit_observer_browsertest.cc.

Bug: 390201794
Change-Id: I691c6c8d26da24247a6c2664ce9072bd5d6ae9df
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6328699
Reviewed-by: Giovanni Ortuno Urquidi <ortuno@chromium.org>
Reviewed-by: Andrew Liu <liu@chromium.org>
Commit-Queue: Svend Larsen <svend@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1429572}