Fix MigrateFlocAllowedToAnnotationsTable test

The MigrateFlocAllowedToAnnotationsTable is testing the existence of
column 'publicly_routable' after the migration; which is incorrect
since the column is removed in version 48.

The CL changes are:
  1) Scoping the database to ensure exclusive opening.
  2) The following assert is removed:
    ASSERT_TRUE(db.DoesColumnExist("visits", "publicly_routable"));

The unittests was passing since the DoesColumnExists doesn't reload
the schema and it is testing the condition on the original (not
 migrated) table.

The column 'publicly_routable is removed in the database version 48.
see: https://source.chromium.org/chromium/chromium/src/+/main:components/history/core/browser/history_database.cc;l=823;drc=ac5fab56b743a8270d71650c536b9e017bc2ca73

For example:
  https://paste.googleplex.com/6581035282726912 (line 28 will fail).

Bug: 1120969
Change-Id: I3b80547ef30803e18667a82e96de268919cf63b6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5096418
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Yao Xiao <yaoxia@chromium.org>
Commit-Queue: Etienne Bergeron <etienneb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1234601}