Import only the implementation of Testing. (pointfreeco#32)

Author: mbrandonw

Sources/MacroTesting/AssertMacro.swift

@@ -10,7 +10,7 @@ import SwiftSyntaxMacros
 import XCTest
 
 #if canImport(Testing)
-  import Testing
+@_implementationOnly import Testing
 #endif
 
 /// Asserts that a given Swift source string matches an expected string with all macros expanded.

Sources/MacroTesting/Internal/RecordIssue.swift

@@ -1,7 +1,7 @@
 import XCTest
 
 #if canImport(Testing)
-  import Testing
+@_implementationOnly import Testing
 #endif
 
 @_spi(Internals)

Sources/MacroTesting/MacrosTestTrait.swift

@@ -2,7 +2,7 @@
   import SnapshotTesting
   import SwiftSyntax
   import SwiftSyntaxMacros
-  import Testing
+  @_implementationOnly import Testing
 
   @_spi(Experimental)
   extension Trait where Self == _MacrosTestTrait {