Deprecate BrowserList::GetLastActive() in tests.

This is a continuing effort to deprecate the incorrect use
of BrowserList::GetLastActive() in browser_tests for verifying
the expected browser being the active one.

Bug: 345848530
Change-Id: I3578b670fab5f901c709b1fb385ffd90e7363652
Include-Ci-Only-Tests: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5619154


Reviewed-by: Erik Chen <erikchen@chromium.org>
Commit-Queue: Jenny Zhang <jennyz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1313703}