Ensure new packages have the correct minimum OS targets when using swift-testing.

swift-testing has a minimum deployment target set higher than the default for packages. We need to ensure that it is respected by `swift package init --enable-experimental-swift-testing`.

Author: grynspan

Sources/Workspace/InitPackage.swift

@@ -186,8 +186,8 @@ public final class InitPackage {
 
             var platforms = options.platforms
 
-            // Macros require macOS 10.15, iOS 13, etc.
-            if packageType == .macro {
+            // Macros and swift-testing require macOS 10.15, iOS 13, etc.
+            if packageType == .macro || options.supportedTestingLibraries.contains(.swiftTesting) {
                 func addIfMissing(_ newPlatform: SupportedPlatform) {
                   if platforms.contains(where: { platform in
                       platform.platform == newPlatform.platform