Use assertThat from Hamcrest instead of JUnit 4

org.junit.Assert.assertThat() is deprecated as of JUnit 4.13.

See gh-22894

spring-oxm/src/test/java/org/springframework/oxm/AbstractMarshallerTests.java

@@ -37,7 +37,7 @@ import javax.xml.transform.stream.StreamResult;
 import java.io.ByteArrayOutputStream;
 import java.io.StringWriter;
 
-import static org.junit.Assert.assertThat;
+import static org.hamcrest.MatcherAssert.assertThat;
 import static org.junit.Assert.assertTrue;
 
 /**

spring-oxm/src/test/java/org/springframework/oxm/jaxb/Jaxb2MarshallerTests.java

@@ -54,9 +54,9 @@ import java.lang.reflect.Method;
 import java.lang.reflect.Type;
 import java.util.Collections;
 
+import static org.hamcrest.MatcherAssert.assertThat;
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertFalse;
-import static org.junit.Assert.assertThat;
 import static org.junit.Assert.assertTrue;
 import static org.junit.Assert.fail;
 import static org.mockito.BDDMockito.eq;

spring-oxm/src/test/java/org/springframework/oxm/xstream/XStreamMarshallerTests.java

@@ -58,6 +58,7 @@ import org.xmlunit.xpath.JAXPXPathEngine;
 
 import org.springframework.util.xml.StaxUtils;
 
+import static org.hamcrest.MatcherAssert.assertThat;
 import static org.junit.Assert.*;
 import static org.mockito.BDDMockito.*;
 import static org.xmlunit.matchers.CompareMatcher.*;