Look in the right place for plugin's classes during integration tests

See gh-9516

spring-boot-tools/spring-boot-gradle-plugin/src/test/java/org/springframework/boot/gradle/testkit/GradleBuild.java

@@ -111,7 +111,7 @@ private void after() {
 	}
 
 	private String pluginClasspath() {
-		return absolutePath("bin") + "," + absolutePath("build/classes/main") + ","
+		return absolutePath("bin") + "," + absolutePath("build/classes/java/main") + ","
 				+ absolutePath("build/resources/main") + ","
 				+ pathOfJarContaining(LaunchScript.class) + ","
 				+ pathOfJarContaining(ClassVisitor.class) + ","