Tests: remove @testable import SwiftFixIt

Use the newly extended APIs for testing and use a non-testable import of
the package, avoiding a recompilation.

Author: compnerd

Tests/SwiftFixItTests/Utilities.swift

@@ -14,7 +14,6 @@ import _InternalTestSupport
 import struct Basics.AbsolutePath
 import var Basics.localFileSystem
 import struct Foundation.UUID
-@testable
 import SwiftFixIt
 import class SwiftSyntax.SourceLocationConverter
 import Testing