Add unit test for cold start tab merge prevention

This CL adds a unit test to verify that no spurious tab merges happen
on cold start if the multi-instance file migration has already run.
The test covers the scenario where the restorable instance count drops
to 0, but the migration preference
`TABMODEL_HAS_RUN_MULTI_INSTANCE_FILE_MIGRATION` is true. It ensures
that `shouldMergeTabs()` returns false and no merge tasks are scheduled.

Bug: 512217702
Change-Id: I294c6ff8712ad928c0f8b6503cc766a1298e5e83
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7846842
Reviewed-by: Calder Kitagawa <ckitagawa@chromium.org>
Reviewed-by: Aishwarya Rajesh <aishwaryarj@google.com>
Commit-Queue: Shu Yang <shuyng@google.com>
Cr-Commit-Position: refs/heads/main@{#1630708}