Use assertThat from Hamcrest instead of JUnit 4

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

See gh-22894

spring-orm/src/test/java/org/springframework/orm/jpa/SharedEntityManagerCreatorTests.java

@@ -28,6 +28,7 @@ import org.junit.runner.RunWith;
 import org.mockito.junit.MockitoJUnitRunner;
 
 import static org.hamcrest.CoreMatchers.*;
+import static org.hamcrest.MatcherAssert.assertThat;
 import static org.junit.Assert.*;
 import static org.mockito.BDDMockito.*;
 import static org.mockito.BDDMockito.verifyNoMoreInteractions;

spring-orm/src/test/java/org/springframework/orm/jpa/persistenceunit/DefaultPersistenceUnitManagerTests.java

@@ -23,8 +23,8 @@ import org.springframework.core.io.ClassPathResource;
 import org.springframework.core.io.DefaultResourceLoader;
 import org.springframework.orm.jpa.domain.Person;
 
+import static org.hamcrest.MatcherAssert.assertThat;
 import static org.hamcrest.Matchers.*;
-import static org.junit.Assert.*;
 
 /**
  * Tests for {@link DefaultPersistenceUnitManager}.