ci: Merge similar try jobs when possible

[mrobinson]

This comes up a lot when triggering wpt-2013 and wpt-2020. Instead of
merging the jobs and triggering both layouts, the try parser will
trigger two separate jobs. Running two of the same builds at once seems
to cause the CI to fail one of them [1]. This fixes that issue.

1. An example of this: https://github.com/servo/servo/actions/runs/7892269495

---

• ./mach build -d does not report any errors
• ./mach test-tidy does not report any errors
• There are tests for these changes

[sagudev]

When triggering WPT for both layouts there is wpt preset, so just creating T-wpt label should run wpt on both layouts. That being said is still think it's good we do merging if possible.

The error you se are due to how artifacts are shared, that I hope will be solved with artifacts-v4 that I am working on.

[sagudev]

This seems to be error-prone when adding new fields in JobConfig. Maybe we could compare them as dictionaries and ignore some fields: https://stackoverflow.com/questions/10480806/compare-dictionaries-ignoring-specific-keys

[mrobinson]

That's an interesting idea, but it seems a bit heavy-weight and people adding new fields to JobConfig will still have to remember to update this function. Instead I've created a new class variable which holds the fields that cannot differ for a merge to happen. This is near the fields and I've added doc to remind people to update the list.

In addition, I've added more tests, which should also help prevent mistakes in the future.

[sagudev]

One suggestion remaining but otherwise looks good.

[mrobinson]

Thanks for the review, @sagudev.