[hotfix] [core] Assert current serializer snapshot is expected snapsh…

…ot class in TypeSerializerSnapshotMigrationTestBase

This strengthens the test base to make use of the provided snapshot
class in the test specifications, to assert that serializers are really
using the expected classes for their current serializer snapshots.

flink-core/src/test/java/org/apache/flink/api/common/typeutils/TypeSerializerSnapshotMigrationTestBase.java

@@ -103,6 +103,8 @@ public void movingForward() throws IOException {
  TypeSerializer<ElementT> restoredSerializer = previousSnapshot.restoreSerializer();
 
  TypeSerializerSnapshot<ElementT> nextSnapshot = restoredSerializer.snapshotConfiguration();
+ assertThat(nextSnapshot, instanceOf(testSpecification.snapshotClass));
+
  TypeSerializerSnapshot<ElementT> nextSnapshotDeserialized = writeAndThenReadTheSnapshot(restoredSerializer, nextSnapshot);
 
  assertThat(nextSnapshotDeserialized, allOf(
@@ -247,6 +249,10 @@ private Path getSnapshotDataLocation() {
  return resourcePath(this.snapshotDataLocation);
  }
 
+ public Class<? extends TypeSerializerSnapshot<T>> getSnapshotClass() {
+ return snapshotClass;
+ }
+
  @Override
  public String toString() {
  return String.format("%s , %s, %s", name, serializerType.getSimpleName(), snapshotClass.getSimpleName());