[waffle]Override the number of search engines using cmd

Override the number of search engines to display a small or large list
using command line arguments.

This will make testing the UI simpler and will deflake some tests.

Screen recording: http://b/329579023#comment2

Fixed: b:329579023
Change-Id: Ife5c16b1f0327228d0eb684fc83ea977b9590648
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5383505


Reviewed-by: Nicolas Dossou-Gbété <dgn@chromium.org>
Auto-Submit: Jack Yammine <jyammine@google.com>
Commit-Queue: Jack Yammine <jyammine@google.com>
Cr-Commit-Position: refs/heads/main@{#1279055}