Fix platforms declaration in IntegrationTests package (#6873)

Not sure why this is only becoming visible now, it seems like this should have been needed since swiftlang/swift-tools-support-core@be6f396

Author: neonichu

IntegrationTests/Package.swift

@@ -4,6 +4,9 @@ import PackageDescription
 
 let package = Package(
     name: "IntegrationTests",
+    platforms: [
+        .macOS(.v10_15),
+    ],
     targets: [
         .testTarget(name: "IntegrationTests", dependencies: [
             .product(name: "SwiftToolsSupport-auto", package: "swift-tools-support-core"),