Investigate new Search Engine configuration

While working on legacy/trac#33533 (moved) I noticed that the engines configured in list.json were being ignored, and had to set the browser.search.modernConfig = false for them to work.

I believe this is because of a new Search Engine configuration that has been enabled by default in nightly recently, this is the meta ticket is https://bugzilla.mozilla.org/show_bug.cgi?id=1542235.

We should investigate this and see whether it's enough to flip that pref to get the behaviour we want.