Add test for nesting of greater blocks.

This shows that greater blocks of different types can be directly nested even if they are both special blocks (as long as they are different special blocks).

org_mode_samples/greater_element/greater_block/nested_special_block.org

@@ -0,0 +1,12 @@
+#+begin_defun
+foo
+#+begin_lorem
+ipsum
+#+end_lorem
+bar
+#+begin_center
+#+begin_quote
+baz
+#+end_quote
+#+end_center
+#+end_defun