Introduce convenience test library APIs for creating test servers for tests in test/jdk/java/net/httpclient

In test/jdk/java/net/httpclient we have a number of tests which create HTTP servers. Many of those tests have repeated code of the form:


        InetSocketAddress sa = new InetSocketAddress(InetAddress.getLoopbackAddress(), 0);
        httpTestServer = HttpTestServer.of(HttpServer.create(sa, 0));
...
        HttpsServer httpsServer = HttpsServer.create(sa, 0);
        httpsServer.setHttpsConfigurator(new HttpsConfigurator(sslContext));
        httpsTestServer = HttpTestServer.of(httpsServer);
...
        http2TestServer = HttpTestServer.of(new Http2TestServer("localhost", false, 0));
...
        https2TestServer = HttpTestServer.of(new Http2TestServer("localhost", true, sslContext));


It would be good to have a convenience API to replace these calls with simpler calls and reduce the boilerplate code in these tests. The new APIs could be of the form:


        httpTestServer = HttpTestServer.create(HTTP_1_1);
...
        httpsTestServer = HttpTestServer.create(HTTP_1_1, sslContext);
...
        http2TestServer = HttpTestServer.create(HTTP_2);
...
        https2TestServer = HttpTestServer.create(HTTP_2, sslContext);


The existing test server API will continue to exist. The proposed new APIs are for the default common cases that are used in most of these tests.