Consider WebAppConfiguration in @SpringBootTest

Update SpringBootTestContextBootstrapper to consider the
`@WebAppConfiguration` annotation and use sensible resourceBasePath
defaults.

Fixes gh-6371

spring-boot-samples/spring-boot-sample-war/src/main/java/sample/war/SampleWarApplication.java

@@ -19,11 +19,13 @@ package sample.war;
 import org.springframework.boot.SpringApplication;
 import org.springframework.boot.autoconfigure.SpringBootApplication;
 import org.springframework.boot.web.support.SpringBootServletInitializer;
+import org.springframework.context.annotation.PropertySource;
 
 /**
  * Sample WAR application
  */
 @SpringBootApplication
+@PropertySource(value = { "WEB-INF/custom.properties" })
 public class SampleWarApplication extends SpringBootServletInitializer {
 
 	public static void main(String[] args) {