Fix bogus designator assignment

[mraspaud]

The designator assignment during TLE parsing was wrong due to an enthusiastic refactoring in 958050ff961fd45ccaed95b1ab7a7bf6692a9a77. This PR restores the functionality and adds a test for it.

• [x] Tests added
• [x] Passes flake8 pyorbital

[codecov[bot]]

Codecov Report

Merging #92 (bf67403) into main (523099d) will increase coverage by 0.19%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #92      +/-   ##
==========================================
+ Coverage   86.67%   86.86%   +0.19%     
==========================================
  Files          13       13              
  Lines        2026     2048      +22     
==========================================
+ Hits         1756     1779      +23     
+ Misses        270      269       -1     

Flag	Coverage Δ
unittests	86.86% <100.00%> (+0.19%)	:arrow_up:

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Impacted Files	Coverage Δ
pyorbital/tests/test_tlefile.py	97.99% <100.00%> (+0.15%)	:arrow_up:
pyorbital/tlefile.py	91.34% <100.00%> (+0.27%)	:arrow_up:

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[mraspaud]

LGTM. Should we also test cases without any platform name within the TLE file?

Test added!