Add a (linked) test case for bug 1108753 (PR 5276)

When submitting PR 5276 there wasn't a good PDF file to include in the test suite. However, with https://bugzilla.mozilla.org/show_bug.cgi?id=1108753, we now have a better source for a test file, hence this patch.
Merge pull request #5532 from Snuffleupagus/test-pr-5276

Add a (linked) test case for bug 1108753 (PR 5276)
Improve fingerprinting of documents

Fixes two issues:
- #4456 : The first 100 bytes are often not unique as they can be
filled with standard PDF headers - so we use the first 200 KB instead.
(This may be overkill)
- Some documents we encountered have invalid xref ids, which were
always coming out as ‘0000000000000000’ - so we detect that and use the
MD5 instead.