Refactor out shared test initialization logic

* Most tests did the exact same setup, so now this will be shared
* Test were inconsistent between using a TabStripModel versus a unique
pointer to one so now they will all use objects of the same type

Change-Id: Iae6e4d6a1170c03fe94631c6c21bf79ce824aeea
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6488775
Reviewed-by: David Pennington <dpenning@chromium.org>
Commit-Queue: Alison Gale <agale@chromium.org>
Reviewed-by: Shibalik Mohapatra <shibalik@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1452991}