Enhanced the test data to demonstrate issue#950

Presence of the "faviconMimeType" attribute in a book entry in library.xml
file is enough for libkiwix to assume that the book contains an illustration
(even if the "favicon" attribute is missing).

test/data/library.xml

@@ -32,6 +32,7 @@
         articleCount="284"
         mediaCount="2"
         size="123"
+        faviconMimeType="image/png;favicon_attribute_is_absent"
       ></book>
   <book
         id="charlesray"
@@ -48,5 +49,6 @@
         articleCount="284"
         mediaCount="2"
         size="556"
+        favicon="faviconMimeType_attribute_is_absent"
       ></book>
 </library>

test/library_server.cpp

@@ -144,7 +144,9 @@ std::string maskVariableOPDSFeedData(std::string s)
   "wikipedia_ru_ray_charles",\
   "",\
   "public_tag_with_a_value:value_of_a_public_tag;_private_tag_with_a_value:value_of_a_private_tag;wikipedia;_pictures:no;_videos:no;_details:no",\
-  "",\
+  "<link rel=\"http://opds-spec.org/image/thumbnail\"\n"          \
+  "          href=\"/ROOT%23%3F/catalog/v2/illustration/raycharles_uncategorized/?size=48\"\n" \
+  "          type=\"image/png;favicon_attribute_is_absent;width=48;height=48;scale=1\"/>\n    ", \
   "zimfile", \
   "zimfile", \
   "125952"\