[ios] Prepare GridMediatorTestClass to work with TabsCloser

Change GridMediatorTestClass to use BrowserWebStateListDelegate and
to configure a mock SceneState. This ensures all tab helpers needed
for BrowserAgent are installed and that moving WebStates between
Browsers will not causes crashes.

Move SetFakePriceDrop() to the only sub-class that needs this method
and change the friend declaration in ShoppingPersistedDataTabHelper
to limit which tests can create fake PriceDrop instances.

Remove BaseGridMediatorTest.TestSelectItemWithPriceDropExperimentOff
which depends on ShoppingPersistedDataTabHelper not being created
when the experiment is off (which is no longer the case).

Bug: 1383087
Change-Id: I6791e832667a14e50294db6c2077ccb4e4496c62
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5059130
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Aliona Dangla <alionadangla@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1228810}