Add test_dropq_diff_vs_util_diff to test_dropq.py

[martinholmer]

Add a test the checks that that difference that generated by the create_difference_table utility function and the dropq_diff_table function have the same values when skipping the actual dropq records-dropping logic.

This pull request involves no change in logic or results, it just adds a new test.

[codecov-io]

Codecov Report

Merging #1531 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #1531   +/-   ##
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  Coverage     100%    100%           
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  Files          37      37           
  Lines        2565    2565           
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  Hits         2565    2565

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