Import only the implementation of Testing. (#903)

Author: mbrandonw

Sources/SnapshotTesting/AssertSnapshot.swift

@@ -1,7 +1,7 @@
 import XCTest
 
 #if canImport(Testing)
-  import Testing
+@_implementationOnly import Testing
 #endif
 
 /// Enhances failure messages with a command line diff tool expression that can be copied and pasted

Sources/SnapshotTesting/Internal/RecordIssue.swift

@@ -1,7 +1,7 @@
 import XCTest
 
 #if canImport(Testing)
-  import Testing
+  @_implementationOnly import Testing
 #endif
 
 var isSwiftTesting: Bool {

Sources/SnapshotTesting/SnapshotsTestTrait.swift

@@ -1,5 +1,5 @@
 #if canImport(Testing)
-  import Testing
+@_implementationOnly import Testing
 
   @_spi(Experimental)
   extension Trait where Self == _SnapshotsTestTrait {