play.test.Helpers.testServer does not use the default test port

The Scala API has a `play.api.test.Helpers.testServerPort` member allowing users to retrieve their test server port from one place (and the value can be overridden by configuration).

The Java API sometimes uses it, but the `play.test.Helpers.testServer` overloads do not use it, forcing users to either always explicitly use their own port number (and not benefiting from the `testServerPort` thing) or to explicitly retrieve the `testServerPort` value using the following expression: `play.api.test.Helpers.testServerPort()`.

We could solve this problem by adding yet another overload of `play.test.Helpers.testServer` that does not take a port number as parameter and retrieves it using `play.api.test.Helpers.testServerPort()`.