Move availability guard to correct test (#734)

It was previously accidentally moved to a different test, which does not need it
Async tests need the guard for macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0

Tests/AsyncHTTPClientTests/HTTPClientTests.swift

@@ -3574,7 +3574,6 @@ final class HTTPClientTests: XCTestCaseHTTPClientTestsBaseClass {
         XCTAssertEqual(.ok, response.status)
     }
 
-    @available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *)
     func testSingletonClientWorks() throws {
         let response = try HTTPClient.shared.get(url: self.defaultHTTPBinURLPrefix + "get").wait()
         XCTAssertEqual(.ok, response.status)
@@ -3586,6 +3585,7 @@ final class HTTPClientTests: XCTestCaseHTTPClientTestsBaseClass {
         }
     }
 
+    @available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *)
     func testAsyncExecuteWithCustomTLS() async throws {
         let httpsBin = HTTPBin(.http1_1(ssl: true))
         defer {