Add missing availability guards to tests (#1874)

Tests/GRPCHTTP2CoreTests/Client/Connection/LoadBalancers/SubchannelTests.swift

@@ -381,6 +381,7 @@ final class SubchannelTests: XCTestCase {
   }
 }
 
+@available(macOS 13.0, iOS 16.0, watchOS 9.0, tvOS 16.0, *)
 extension ConnectionBackoff {
   static func fixed(at interval: Duration, jitter: Double = 0.0) -> Self {
     return Self(initial: interval, max: interval, multiplier: 1.0, jitter: jitter)

Tests/GRPCHTTP2CoreTests/Client/Connection/Utilities/TestServer.swift

@@ -22,6 +22,7 @@ import NIOPosix
 
 @testable import GRPCHTTP2Core
 
+@available(macOS 13.0, iOS 16.0, watchOS 9.0, tvOS 16.0, *)
 final class TestServer: Sendable {
   private let eventLoopGroup: any EventLoopGroup
   private typealias Stream = NIOAsyncChannel<RPCRequestPart, RPCResponsePart>