In https://bugzilla.mozilla.org/show_bug.cgi?id=1577452 there are some tests that are skipped because they changed cache behaviour, need to reenable them with modernconfig

The test as originally written, relied on the fact that the search service would install only the engines listed in the engines array within the cache, and nothing else.

The test was later semi-broken, as it wasn't starting the add-on manager which meant the built-in engines weren't being handled.

Finally, we made changes recently to ensure that the built-in engines are installed if they aren't listed in the cache. This test didn't fail as a result of those changes due to being broken where we'd missed starting the add-on manager.

The changes here make it use the shorter 2-engine list of engines, and include those in the cache reading/writing. This makes it work as expected.

Depends on D53185

Depends on D53186

This makes visibleDefaultEngines in the cache redundant, and replaces it with a new builtInEngineList that is indexed via the id and locale of the WebExtension. This gives a better indexing method that just display name.

Also bump the cache version to 2, so that we can do safe downgrades/upgrades with pref flips that might not happen at the time of an upgrade.

Depends on D53188

https://hg.mozilla.org/mozilla-central/rev/9c3e3897366f
https://hg.mozilla.org/mozilla-central/rev/af4f34d0ca4a
https://hg.mozilla.org/mozilla-central/rev/39e39bb24898
https://hg.mozilla.org/mozilla-central/rev/5c271dd2d41a