Add test for JBIG2 decoder with >4 referred-to segments

Add regression test for issue #20439 to ensure that JBIG2 images
with more than 4 referred-to segments are handled correctly.

Author: Gaurang-5

test/pdfs/issue20439.pdf

@@ -2809,6 +2809,13 @@
     "rounds": 1,
     "type": "eq"
   },
+  {
+    "id": "issue20439",
+    "file": "pdfs/issue20439.pdf",
+    "md5": "3c7e888b26ff00943ec1610d93235efc",
+    "rounds": 1,
+    "type": "eq"
+  },
   {
     "id": "issue15942",
     "file": "pdfs/issue15942.pdf",