Improve tests: Add pretty_org fixtures to allow testing pretty printing

Until now we expected the .org file to print back to itself - we can't do that
when the input is not pretty printed already - with the introduction of blocks
with unindented content that will be the case.

org/testdata/footnotes.pretty_org

@@ -0,0 +1,28 @@
+* Using some footnotes
+- normal footnote reference [fn:1] [fn:6]
+- further references to the same footnote should not [fn:1] render duplicates in the footnote list
+- inline footnotes are also supported via [fn:2:the inline footnote definition].
+
+* Footnotes
+[fn:1] https://www.example.com
+- footnotes can contain *markup*
+- and other elements
+  - like blocks
+    #+BEGIN_SRC
+    other non-plain
+    #+END_SRC
+  - and tables
+    | 1 | a |
+    | 2 | b |
+    | 3 | c |
+
+[fn:3] [[http://example.com/unused-footnote][example.com/unused-footnote]]
+
+[fn:4] another unused footnote
+
+[fn:5] another unused footnote
+
+[fn:6]
+
+Footnotes break after two consecutive empty lines - just like paragraphs - see https://orgmode.org/worg/dev/org-syntax.html.
+This shouldn't happen when the definition line and the line after that are empty.