Force-enable kTabPickupThreshold in TabPickupSettingsTestCase

This was accidentally removed in [1].

[1] https://chromium-review.googlesource.com/c/chromium/src/+/5388655/12/ios/chrome/browser/ui/settings/tabs/tab_pickup/tab_pickup_settings_egtest.mm@90

Bug: 331464076
Change-Id: I948a2fb71c728c0ebe2d53ae939140436afac18f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5401653


Reviewed-by: Joemer Ramos <joemerramos@chromium.org>
Commit-Queue: Joemer Ramos <joemerramos@chromium.org>
Auto-Submit: Victor Vianna <victorvianna@google.com>
Commit-Queue: Victor Vianna <victorvianna@google.com>
Owners-Override: Joemer Ramos <joemerramos@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1278925}