Use Gradle test fixture support for spring-core

See gh-23550

spring-messaging/src/test/java/org/springframework/messaging/MessageHeadersTests.java

@@ -25,7 +25,7 @@ import java.util.concurrent.atomic.AtomicLong;
 
 import org.junit.jupiter.api.Test;
 
-import org.springframework.util.SerializationTestUtils;
+import org.springframework.core.test.fixtures.io.SerializationTestUtils;
 
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.api.Assertions.assertThatIllegalArgumentException;

spring-messaging/src/test/java/org/springframework/messaging/converter/MarshallingMessageConverterTests.java

@@ -25,10 +25,10 @@ import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 import org.xmlunit.diff.DifferenceEvaluator;
 
+import org.springframework.core.test.fixtures.xml.XmlContent;
 import org.springframework.messaging.Message;
 import org.springframework.messaging.support.MessageBuilder;
 import org.springframework.oxm.jaxb.Jaxb2Marshaller;
-import org.springframework.tests.XmlContent;
 
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.api.Assertions.assertThatExceptionOfType;

spring-messaging/src/test/java/org/springframework/messaging/handler/invocation/reactive/TestEncoderMethodReturnValueHandler.java

@@ -24,7 +24,7 @@ import org.springframework.core.MethodParameter;
 import org.springframework.core.ReactiveAdapterRegistry;
 import org.springframework.core.codec.Encoder;
 import org.springframework.core.io.buffer.DataBuffer;
-import org.springframework.core.io.buffer.support.DataBufferTestUtils;
+import org.springframework.core.test.fixtures.io.buffer.DataBufferTestUtils;
 import org.springframework.messaging.Message;
 
 import static java.nio.charset.StandardCharsets.UTF_8;

spring-messaging/src/test/java/org/springframework/messaging/rsocket/LeakAwareNettyDataBufferFactory.java

@@ -31,7 +31,7 @@ import org.springframework.core.io.buffer.PooledDataBuffer;
 import org.springframework.util.ObjectUtils;
 
 /**
- * Unlike {@link org.springframework.core.io.buffer.LeakAwareDataBufferFactory}
+ * Unlike {@link org.springframework.core.test.fixtures.io.buffer.LeakAwareDataBufferFactory}
  * this one is an instance of {@link NettyDataBufferFactory} which is necessary
  * since {@link PayloadUtils} does instanceof checks, and that also allows
  * intercepting {@link NettyDataBufferFactory#wrap(ByteBuf)}.

spring-messaging/src/test/java/org/springframework/messaging/rsocket/MetadataEncoderTests.java

@@ -33,7 +33,7 @@ import org.springframework.core.io.buffer.DataBuffer;
 import org.springframework.core.io.buffer.DefaultDataBufferFactory;
 import org.springframework.core.io.buffer.NettyDataBuffer;
 import org.springframework.core.io.buffer.NettyDataBufferFactory;
-import org.springframework.core.io.buffer.support.DataBufferTestUtils;
+import org.springframework.core.test.fixtures.io.buffer.DataBufferTestUtils;
 import org.springframework.util.MimeType;
 import org.springframework.util.MimeTypeUtils;
 

spring-messaging/src/test/java/org/springframework/messaging/rsocket/PayloadUtilsTests.java

@@ -30,7 +30,7 @@ import org.springframework.core.io.buffer.DataBufferUtils;
 import org.springframework.core.io.buffer.DefaultDataBuffer;
 import org.springframework.core.io.buffer.DefaultDataBufferFactory;
 import org.springframework.core.io.buffer.NettyDataBuffer;
-import org.springframework.core.io.buffer.support.DataBufferTestUtils;
+import org.springframework.core.test.fixtures.io.buffer.DataBufferTestUtils;
 
 import static org.assertj.core.api.Assertions.assertThat;
 

spring-messaging/src/test/java/org/springframework/messaging/support/MessageHeaderAccessorTests.java

@@ -24,10 +24,10 @@ import java.util.UUID;
 
 import org.junit.jupiter.api.Test;
 
+import org.springframework.core.test.fixtures.io.SerializationTestUtils;
 import org.springframework.messaging.Message;
 import org.springframework.messaging.MessageHeaders;
 import org.springframework.util.MimeTypeUtils;
-import org.springframework.util.SerializationTestUtils;
 
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.api.Assertions.assertThatIllegalStateException;