Add some tests for db and core modules

[nadege]

Cf. #29 but there is still work to do

Successful PR Checklist:

• Tests

[codecov]

Codecov Report

Merging #62 into master will increase coverage by 2.75%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #62      +/-   ##
==========================================
+ Coverage   64.93%   67.69%   +2.75%     
==========================================
  Files          14       14              
  Lines         616      616              
  Branches       86       86              
==========================================
+ Hits          400      417      +17     
+ Misses        208      191      -17     
  Partials        8        8              

Flag	Coverage Δ
#integration	45.29% <ø> (ø)
#unit	57.62% <ø> (+3.08%)	⬆️

Impacted Files	Coverage Δ
septentrion/configuration.py	82.35% <0.00%> (+1.47%)	⬆️
septentrion/db.py	67.18% <0.00%> (+25.00%)	⬆️

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