Refactor GenBank tests

[Andrey-Raspopov]

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Good morning!

I've taken several attempts at refactoring GenBank library, but it is truly a complex task, so I've decided to start from the other end. This commit is focused on moving test data away from the test logic and of course is a subject of a discussion.

[codecov]

Codecov Report

Merging #3291 into master will not change coverage.
The diff coverage is n/a.

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  Coverage   83.98%   83.98%           
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  Files         318      318           
  Lines       51660    51660           
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  Hits        43389    43389           
  Misses       8271     8271           

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[peterjc]

What do you think @mdehoon? We started with a print-and-compare test, where a string representation the expected parser output was held in a text file. Then in #2124 you refactored to use the unittest framework with the expected parser output as python datatypes. Here it moves the expected output externally again, but into a JSON file.

If I was doing this from scratch, I'd probably start with a single file where all the parsed values are verified, supplemented by specific tests of corner cases (e.g. strange line wrapping, value-less qualifiers, atypical line breaks in structured comments, expected warnings from bad locations, bad quotes, ...).

(Update: fixed typo)