Add default for swiftTestingVersionForTestTemplate

Author: DougGregor

Sources/PackageModel/InstalledSwiftPMConfiguration.swift

@@ -31,7 +31,8 @@ public struct InstalledSwiftPMConfiguration: Codable {
 
     let version: Int
     public let swiftSyntaxVersionForMacroTemplate: Version
-    public let swiftTestingVersionForTestTemplate: Version
+    public var swiftTestingVersionForTestTemplate: Version =
+        `default`.swiftTestingVersionForTestTemplate
 
     public static var `default`: InstalledSwiftPMConfiguration {
         return .init(