As of Spring Framework 5.3, `WebTestClient` can now be configured on top
of `MockMvc` for testing Spring MVC applications in a mock environment.
Prior to this commit, `WebTestClient` would be already configured for
WebFlux mock setups with `@AutoConfigureWebTestClient` or live servers
(for both MVC and WebFlux apps).
This commit enhances the `@AutoConfigureWebMvc` support so that a
`WebTestClient` instance is auto-configured if the spring-webflux
dependency is present on the classpath.
Closes gh-23067
8b3bea173c