refactor: AssertJ best practices

Use this link to re-run the recipe: https://app.moderne.io/recipes/org.openrewrite.java.testing.assertj.Assertj?organizationId=T3BlblJld3JpdGU%3D

Co-authored-by: Moderne <team@moderne.io>

src/test/java/org/openrewrite/java/security/xml/ExternalDTDAccumulatorTest.java

@@ -21,13 +21,13 @@
 
 import java.util.stream.Stream;
 
-import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.assertj.core.api.Assertions.assertThat;
 
 class ExternalDTDAccumulatorTest {
     @ParameterizedTest
     @MethodSource("provideEntitySplitTestArguments")
     void entitySplit(String initial, String expected) {
-        assertEquals(expected, ExternalDTDAccumulator.extractURLFromEntity(initial));
+        assertThat(ExternalDTDAccumulator.extractURLFromEntity(initial)).isEqualTo(expected);
     }
 
     private static Stream<Arguments> provideEntitySplitTestArguments() {