Make sure that failsafe uses target/classes

This commit configures the failsafe plugin to use the classes directory
rather than the produced jar file as the latter can be a repackaged jar
by default.

Closes gh-11974

spring-boot-project/spring-boot-starters/spring-boot-starter-parent/pom.xml

@@ -87,6 +87,9 @@
 							</goals>
 						</execution>
 					</executions>
+					<configuration>
+						<classesDirectory>${project.build.outputDirectory}</classesDirectory>
+					</configuration>
 				</plugin>
 				<plugin>
 					<groupId>org.apache.maven.plugins</groupId>