This commit replaces the option to bind the WebTestClient to an
HttpHandler to bind to a WebHandler instead.
This allows testing below the WebFlux level such as WebFilter,
WebHandler, or WebSession scenarios, but still a level above
HttpHandler so that WebTestClient is in charge of creating the
ServerWebExchange and expose consistently the
WebTestClient#MockServerSpec setup across all "mock" server bindToXxx
options.
Issue: SPR-15570