Improve testing with WebClient

[mdeinum]

When we are using a RestTemplate or RestClient we can easily write tests using the MockRestServiceServer and register some behavior. For more complex testing scenarios it is possible to use things like wiremock etc.

For the reactive version of the web there is no equivalent MockRestServiceServer, so when testing the WebClient we would always need something like wiremock or another library or totally mock the dependency.

It would be nice if there would be an equivalent way of of testing WebClient based code like there is for RestTemplate/RestClient.

[snicoll]

Duplicate of #19852