fix: add missing Mockito when() import in DorisSinkTest

The test used Mockito.when() without the corresponding static import,
causing compilation failure on CI.

Author: gnehil

flink-doris-connector/flink-doris-connector-flink1/src/test/java/org/apache/doris/flink/sink/DorisSinkTest.java

@@ -43,6 +43,7 @@
 import static org.mockito.ArgumentMatchers.any;
 import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.mockStatic;
+import static org.mockito.Mockito.when;
 
 public class DorisSinkTest {