This adds tab-to-search telemetry, both for the new tabtosearch search mode entry point and for tabtosearch results in our usual Urlbar result-selection scalars. I also added a subtest in browser_urlbar_event_telemetry, but realized as I was writing it that it was not useful. We don't consider entering search more as the end of an engagement, so tab-to-search results will not appear in event telemetry. We already considered this in bug 1654680 and resolved it by adding detailed urlbar.searchmode.* scalars, so I don't consider it a blocker. I left the new subtest in since it was mostly done anyways and it can't hurt.

Differential Revision: https://phabricator.services.mozilla.com/D91469