Add Swix to the Swift Source Compatibility test suite

[felix91gr]

Here it is.

These tests should be good enough. I think.

Edit

Ah, I see. You're working now on the package-manager branch. Take your time... :)
Maybe I'll drop a PR to help you implementing/porting/testing stuff ^^

[stsievert]

I would love help with the port/with the package-manager branch.

Looking at the requirements for inclusion to the Source Compatibility:

Have maintainers who will commit to resolve issues in a timely manner

I can't commit to this.

[stsievert]

The package-manager branch is moving to Swix 3 and refactoring a bit (renaming vector to Vector, etc)

[stsievert]

Well, and also moving to the Swift Package Manager.