Add a *linked* test-case for issue 2618

Given that this PDF document is an interesting test-case for performance reasons, w.r.t. inline image caching, it probably can't hurt to add it to the test-suite to make it more readily available.
Considering the contents of that PDF document I'm not sure if we can include it directly in the repository, hence why a *linked* test-case was choosen here.

test/pdfs/issue2618.pdf.link

@@ -0,0 +1 @@
+https://github.com/mozilla/pdf.js/files/9983741/issue2618.pdf

test/test_manifest.json

@@ -3047,6 +3047,14 @@
        "rounds": 1,
        "type": "eq"
     },
+    {  "id": "issue2618",
+       "file": "pdfs/issue2618.pdf",
+       "md5": "2c554a99a52288ca1a44a422eeafb8fb",
+       "rounds": 1,
+       "lastPage": 1,
+       "link": true,
+       "type": "eq"
+    },
     {  "id": "issue2642",
       "file": "pdfs/issue2642.pdf",
       "md5": "b6679861fdce3bbab0c1fa51bb7f5077",