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.

etc/generate-fixtures

@@ -2,4 +2,5 @@
 
 for org_file in org/testdata/*.org; do
     ./go-org $org_file html > org/testdata/$(basename $org_file .org).html
+    ./go-org $org_file org > org/testdata/$(basename $org_file .org).pretty_org
 done