Since SPR-15536, WebFlux.fn is now configured with `@EnableWebFlux`,
along the annotated controllers. Both `RouterFunction` and Controller
instances can now live within the same application and they share
the same web infrastructure.
This commit removes the custom auto-configuration for `RouterFunction`
and relies on `@EnableWebFlux` for that.
Closes gh-9165
This commit exposes a `WebTestClient` automatically in a reactive
integration test that uses an embedded web server. This is similar to
what we do with `TestRestTemplate` for servlet based integration tests.
Closes gh-8399
