Add end to end test for the geographic gatherer

[mraspaud]

Just adds an end to end test: send in messages and check the output message.

[codecov[bot]]

Codecov Report

Merging #106 (1390eb7) into main (9cf5182) will increase coverage by 0.58%. The diff coverage is 98.64%.

:exclamation: Current head 1390eb7 differs from pull request most recent head 3bb827e. Consider uploading reports for the commit 3bb827e to get more accurate results

@@            Coverage Diff             @@
##             main     #106      +/-   ##
==========================================
+ Coverage   89.58%   90.17%   +0.58%     
==========================================
  Files          25       25              
  Lines        3609     3673      +64     
==========================================
+ Hits         3233     3312      +79     
+ Misses        376      361      -15     

Impacted Files	Coverage Δ
pytroll_collectors/geographic_gatherer.py	87.96% <83.33%> (+2.47%)	:arrow_up:
...troll_collectors/tests/test_geographic_gatherer.py	98.50% <100.00%> (+0.45%)	:arrow_up:
pytroll_collectors/triggers/_base.py	88.97% <0.00%> (+9.44%)	:arrow_up:

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.