Update Spring MVC Test reference
Update Spring MVC Test reference

Add section on Spring MVC TEst vs full integation testing and provide
reference to Spring Boot's @WebIntegrationTest as an alternative.

Issue: SPR-13169

spring-projects/spring-framework,spring-projects/spring-framework,spring-projects/spring-framework,spring-projects/spring-framework,spring-projects/spring-framework,spring-projects/spring-framework,spring-projects/spring-framework