refactor(test): expose internal class

Explicitly exposes ExporterMappedObject, as it's already implicitly
accessible/exposed via the `setJson` methods that its public sub-classes
inherit.

(cherry picked from commit 354b139)

test/src/main/java/io/camunda/zeebe/test/exporter/record/ExporterMappedObject.java

@@ -10,7 +10,7 @@
 import com.fasterxml.jackson.annotation.JsonIgnore;
 import io.camunda.zeebe.broker.exporter.ExporterObjectMapper;
 
-class ExporterMappedObject {
+public class ExporterMappedObject {
 
   protected static final ExporterObjectMapper OBJECT_MAPPER = new ExporterObjectMapper();