Issue 6854 - Re-enable testing of 'aboutstacks.pdf'

This test was disabled in PR 4732, because the file was no longer available. The motivation being that there were two other files which should be good replacements. However, one of those has since been replaced with a reduced test-case (which doesn't exercise the same code-path), and in the other one the error does not appear to be entirely identical.
Hence it seems reasonable to re-add the 'aboutstacks.pdf' test, since it was possible to find it on the Internet Archive (by searching using a different URL, compared to the current one).

test/pdfs/aboutstacks.pdf.link

@@ -1 +1 @@
-http://greenhousechallenge.org/media/item/313/38/About-Stacks.pdf
+http://web.archive.org/web/20160112115354/http://www.fao.org/fileadmin/user_upload/tci/docs/2_About%20Stacks.pdf

test/test_manifest.json

@@ -978,6 +978,14 @@
       "rounds": 1,
       "type": "load"
     },
+    {  "id": "aboutstacks",
+       "file": "pdfs/aboutstacks.pdf",
+       "md5": "6e7c8416a293ba2d83bc8dd20c6ccf51",
+       "rounds": 1,
+       "link": true,
+       "lastPage": 1,
+       "type": "eq"
+    },
     {  "id": "smaskdim",
       "file": "pdfs/smaskdim.pdf",
       "md5": "de80aeca7cbf79940189fd34d59671ee",