Use Gradle test fixture support for spring-core

See gh-23550

spring-aspects/spring-aspects.gradle

@@ -24,6 +24,7 @@ dependencies {
 	optional("javax.transaction:javax.transaction-api")  // for @javax.transaction.Transactional support
 	testCompile(project(":spring-core"))  // for CodeStyleAspect
 	testCompile(project(":spring-test"))
+	testCompile(testFixtures(project(":spring-core")))
 	testCompile("javax.mail:javax.mail-api")
 	testCompileOnly("org.aspectj:aspectjrt")
 }