Add missing available annotation to streaming tests (#33)

### Motivation

The tests had a missing `@available` annotation which meant that they
didn't build iOS (annoyingly, when building for macOS, these things are
not checked so things were building fine locally).

### Modifications

Add missing available annotation to streaming tests.

### Result

Builds for iOS again.

### Test Plan

Local testing, selecting generic iOS destination in Xcode. This failed
before this patch, and succeeds with it.

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

Author: simonjbeaumont

Tests/OpenAPIURLSessionTests/URLSessionBidirectionalStreamingTests/URLSessionBidirectionalStreamingTests.swift

@@ -21,7 +21,7 @@ import OpenAPIRuntime
 import XCTest
 @testable import OpenAPIURLSession
 
-class URLSessionBidirectionalStreamingTests: XCTestCase {
+@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *) class URLSessionBidirectionalStreamingTests: XCTestCase {
     // swift-format-ignore: AllPublicDeclarationsHaveDocumentation
     static override func setUp() { OpenAPIURLSession.debugLoggingEnabled = false }