Add option to group @testable imports at the top or bottom

[fdiaz]

This PR adds a config option to group @testable imports at the top or bottom when sorting them.

This way instead of:

import Bar
@testable import Foo
import UIKit

we could get:

import Bar
import UIKit
@testable import Foo

or:

@testable import Foo
import Bar
import UIKit

This is a style I've seen in some projects, putting the @testable at the bottom so it's easier to know what module is being tested.

This is my first PR to SwiftFormat so I'm more than happy to revisit anything.

[nicklockwood]

@fdiaz thanks, this looks great!

The argument names are a bit verbose compared with the rest of the project, although I don't have suggestions for better ones right now. That is contributing to the test failure in the help-line-length test though.

I'll do a more thorough review later when I get a chance.

[nicklockwood]

Equatable is redundant here, I think, since String already conforms to Equatable.

[fdiaz]

How do you feel about "normal"/"sorted" instead of "alphabetically"?

[fdiaz]

I'm gonna go with alphabetized for now instead of alphabetically. I think that makes it exactly 80. Let me know if you think there's a better word and I can change it 😉

[coveralls]

Coverage increased (+0.04%) to 89.618% when pulling e177b06 on fdiaz:sorted-imports-testable into 0bef4e4 on nicklockwood:develop.

[nicklockwood]

Lgtm. Thanks!