Update the file used with the xfa_bug1720182 test-case

The file used in this test-case is *identical* to, i.e. the md5 entry perfectly matches, the file used with the `xfa_bug1716380` test-case.
While it's obviously fine to use the same PDF document in different reference-tests, note how we e.g. have both `eq` and `text` tests for one document, we should always avoid adding *duplicate* files in the `test/pdfs/` folder.

test/pdfs/xfa_bug1720182.pdf.link

@@ -1 +0,0 @@
-https://bugzilla.mozilla.org/attachment.cgi?id=9230780

test/test_manifest.json

@@ -1157,7 +1157,7 @@
        "type": "eq"
     },
     {  "id": "xfa_bug1720182",
-       "file": "pdfs/xfa_bug1720182.pdf",
+       "file": "pdfs/xfa_bug1716380.pdf",
        "md5": "1351f816f0509fe750ca61ef2bd40872",
        "link": true,
        "rounds": 1,
@@ -1174,7 +1174,8 @@
          "ComplainantLastname2711": {
            "value": "Bar"
          }
-       }
+       },
+       "about": "This *intentionally* uses the same file as test-case xfa_bug1716380."
     },
     {  "id": "bug1720411",
        "file": "pdfs/bug1720411.pdf",