Add missing avaliable annotations to test code (#1720)

Signed-off-by: Si Beaumont <beaumont@apple.com>

Tests/GRPCCoreTests/Call/Client/Internal/ClientRPCExecutorTestSupport/ClientRPCExecutorTestHarness+Transport.swift

@@ -17,6 +17,7 @@ import Atomics
 
 @testable import GRPCCore
 
+@available(macOS 13.0, iOS 16.0, watchOS 9.0, tvOS 16.0, *)
 extension InProcessServerTransport {
   func spawnClientTransport(
     throttle: RetryThrottle = RetryThrottle(maximumTokens: 10, tokenRatio: 0.1)

Tests/GRPCCoreTests/Call/Client/Internal/ClientRPCExecutorTests+Hedging.swift

@@ -16,6 +16,7 @@
 import GRPCCore
 import XCTest
 
+@available(macOS 13.0, iOS 16.0, watchOS 9.0, tvOS 16.0, *)
 extension ClientRPCExecutorTests {
   func testHedgingWhenAllAttemptsResultInNonFatalCodes() async throws {
     let harness = ClientRPCExecutorTestHarness(
@@ -185,6 +186,7 @@ extension ClientRPCExecutorTests {
   }
 }
 
+@available(macOS 13.0, iOS 16.0, watchOS 9.0, tvOS 16.0, *)
 extension ClientRPCExecutionConfiguration {
   fileprivate static func hedge(
     maximumAttempts: Int = 5,

Tests/GRPCCoreTests/Transport/InProcessClientTransportTests.swift

@@ -18,6 +18,7 @@ import XCTest
 
 @testable import GRPCCore
 
+@available(macOS 13.0, iOS 16.0, watchOS 9.0, tvOS 16.0, *)
 final class InProcessClientTransportTests: XCTestCase {
   struct FailTest: Error {}