Bug 1930985 - Implement search engine order handling on the Rust based SearchEngineSelector.

[mandysGit]

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Branch builds: add [firefox-android: branch-name] to the PR title.

[mandysGit]

This PR is still work in progress and there will be more changes soon.
I need a first pass review, please.

I've added the implemention for orderHint property. I included setting the sort priority and sorting the engines orders.

The part I need some feedback on right now is the first test I wrote, and of course everything else if there's any suggestions.

I converted this test into Rust in the selector.rs:

https://searchfox.org/mozilla-central/source/toolkit/components/search/tests/xpcshell/test_engine_selector_engine_orders.js#163-173

And I'm copying over this engine config from the top of the file:

https://searchfox.org/mozilla-central/source/toolkit/components/search/tests/xpcshell/test_engine_selector_engine_orders.js#15-43

As I was following the test structure in selector.rs I noticed the config and expected_engines parts are duplicated and very long. The way we've done it in Desktop is that we hadn't need to write all of the properties in. I wanted some feedback if I should continue the tests in this format even though they're quite long and has lots of duplication?

My aim is to continue writing tests for starts_with and defaults engines. But that would mean I'll need to create another json config and another expected_engines list -- which I think is fine to do but there's a lot of duplication.

Curious to know what your thoughts on this are?

[Standard8]

There's a piece missing here, which the engine selector on desktop doesn't do - the items which aren't default/have an order hint set, need to be sorted alphabetically by their display name.

This is the full function we use: https://searchfox.org/mozilla-central/rev/5b061cdc4d40d44988dc61aa941cfbd98e31791f/toolkit/components/search/SearchUtils.sys.mjs#362-440

One thing I'm not sure about is how we'll handle the locale-based sorting like we do on desktop. I haven't looked into the options for sort functions on Rust.

[Standard8]

I'm tempted to say we could put the sort related functions into a separate module, which might make it easier to write unit tests for them (and smaller module sizes), what do you think?