Add missing availability annotations in tests (#1813)

Tests/GRPCHTTP2CoreTests/Client/Resolver/NameResolverRegistryTests.swift

@@ -18,6 +18,7 @@ import GRPCCore
 import GRPCHTTP2Core
 import XCTest
 
+@available(macOS 13.0, iOS 16.0, watchOS 9.0, tvOS 16.0, *)
 final class NameResolverRegistryTests: XCTestCase {
   struct FailingResolver: NameResolverFactory {
     typealias Target = StringTarget