Add failing test for dumping simple1.pdf and simple3.pdf, because the…

[pietermarsman]

Pull request

This PR removes the trailer output of the fallback xref from dumppdf.py by default, and add a flag to enable this behavior. In most cases the PDF has one or more valid xrefs and showing those is enough and expected. Showing the fallback xref is only usefull when the PDF does not have an xref.

Fixes #176

How Has This Been Tested?

Ran test, tested on commandline.

Checklist

• I have added tests that prove my fix is effective or that my feature
  works
• I have added docstrings to newly created methods and classes
• I have optimized the code at least one time after creating the initial
  version
• I have updated the README.md or I am verified that this
  is not necessary
• I have updated the readthedocs documentation or I
  verified that this is not necessary
• I have added a consice human-readable description of the change to
  CHANGELOG.md

[wlbentley]

I can only offer a high-level review of the code, but with the flag and the informative Warning message, it looks like it covers all use cases. Big improvement. Thanks.