Update test to use 4.x Junit API

JAVA-4219

Author: jyemin

driver-core/src/test/unit/com/mongodb/MongoWriteExceptionTest.java

@@ -21,7 +21,8 @@
 import org.bson.BsonInt32;
 import org.junit.Test;
 
-import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.Assert.assertEquals;
+
 
 public class MongoWriteExceptionTest {