This commit add mock support for WebFlux with an infrastructure similar
to what `WebMvcTest` provides. `@WebFluxTest` can be used to test
controllers with a narrowed classpath that is relevant to WebFlux. Also,
`@SpringBootTest` now starts WebFlux in "mock" mode by default and
`@AutoConfigureWebTestClient` can be used to inject a `WebTestClient`
connected to the `ApplicationContext`.

To make that happen, a `ReactiveWebApplicationContext` interface has been
introduced to mirror what `WebApplicationContext` currently does. Things
are still a bit volatile at this point and that infra may move to Spring
Framework at some point.

Closes gh-8401