Previously, tests annotated with @WebMvcTest would use Spring
Framework’s WebDelegatingSmartContextLoader. This meant that
SpringApplication was not used to create the application context. One
effect of this was that Spring Boot’s logging system was not initialized
and large quantities of debug log output was produced due to Logback’s
default behaviour.

This commit updates WebMvcTestContextBootstrapper so that it uses
SpringBootContextLoader. This aligns the behaviour of @WebMvcTest
with the behaviour of @SpringBootTest and @DataJpaTest.

Closes gh-5847