[PWA/Nav Capturing] Tests for focus existing open in tab PWAs

This CL adds automated testing for the use-case where the initial app
being launched via redirection can open in a new PWA window or a tab,
while the final destination app is set to always launch in a tab, with
both apps set to focus-existing.

The behavior should be such that no new tab for app B is opened, and
any intermediary app windows that are opened are taken care of
accordingly.

The test suite is flaky when it comes to waiting for a web contents
to be destroyed properly, so this CL reuses an utility function that
after a navigation, iterates through all web contents and waits for it
to either finish loading or destroy itself before performing any
assertions, reducing flakiness.

Bug: 359870039, 351775835
Change-Id: I68fb2b962b8ffb4df150995130aaf11bf373533a
Fixed: 375317754
Include-Ci-Only-Tests: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5961116
Reviewed-by: Daniel Murphy <dmurph@chromium.org>
Commit-Queue: Dibyajyoti Pal <dibyapal@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1374481}