Add missing test for aero_background

[jcurtis2]

Closes #223 by adding a test for getting an AeroDist of the background.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.69%. Comparing base (da95a8d) to head (8b136ca).
⚠️ Report is 21 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #459   +/-   ##
=======================================
  Coverage   90.69%   90.69%           
=======================================
  Files          55       55           
  Lines        2537     2537           
  Branches      144      144           
=======================================
  Hits         2301     2301           
  Misses        163      163           
  Partials       73       73           

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