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/pom.xml

@@ -43,6 +43,11 @@
 				</exclusion>
 			</exclusions>
 		</dependency>
+		<dependency>
+			<groupId>org.springframework.boot</groupId>
+			<artifactId>spring-boot-starter-test</artifactId>
+			<scope>test</scope>
+		</dependency>
 	</dependencies>
 	<build>
 		<plugins>