test: make ConverterTest class public

Author: delei

fesod-sheet/src/test/java/org/apache/fesod/sheet/converter/ConverterTest.java

@@ -78,7 +78,7 @@
  * Unit tests for individual converter implementations.
  */
 @Tag(Tags.UNIT)
-class ConverterTest {
+public class ConverterTest {
 
     private static final GlobalConfiguration GLOBAL_CONFIGURATION = new GlobalConfiguration();