Add a public initializer for Testing.Test

[younata]

Description

I'm in the process of creating a BDD-style DSL on top of Swift Testing (https://github.com/Quick/Quick/pull/1292), and I'm looking at creating Testing.Tests. I could use Testing.Test.__function, but that's explicitly discouraged (though that's definitely what I'll be using for the time being).

I'd like for the Test struct initializers to be made public, so that I have an explicitly supported way to generate Test objects.