[FLINK-35011][tests] Change MockDeserializationSchema's visibility ba…

…ck to public to prevent compilation failure in external connectors

flink-streaming-java/src/test/java/org/apache/flink/streaming/util/MockDeserializationSchema.java

@@ -33,7 +33,7 @@
  *
  * <p>It does not implement any of the deserialization methods.
  */
-class MockDeserializationSchema<T> implements DeserializationSchema<T> {
+public class MockDeserializationSchema<T> implements DeserializationSchema<T> {
 
     private boolean openCalled = false;