Add multiple ``CdxServerDedup`` unit tests to simulate found, not found and
invalid responses from the CDX server. Use a different file
``tests/test_dedup.py`` because we test the CdxServerDedup component
individually and it belongs to the ``warcprox.dedup`` package.
Add ``mock`` package to dev requirements.
Rework the warcprox.dedup.CdxServerDedup class to have better exception
handling.