Add test coverage to `bookmarks_file_importer`.

This commit adds test coverage to `bookmarks_file_importer` to cover
different use cases.

The tests covers:
* Importing bookmarks.
* Import empty bookmarks file.
* Import bookmarks with invalid entries.
* Import search engine.
* Import bookmarks with favicons.

Bug: 430312165
Change-Id: I804f7cf28d6f9aee20ff7939cea45a9fffd7c7ad
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6818983
Auto-Submit: Michael Tatarski <mtatarski@google.com>
Commit-Queue: Michael Tatarski <mtatarski@google.com>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1497598}