test: Update test cases to get full coverage

[effigies]

I wanted to test the EAGER_IMPORT functionality, and figured we could cover these last few missing lines.

I did remove a guard in DelayedImportErrorModule.__getattr__ that coverage showed was never being entered. That method does not need a super() call, since __class__, __frame_data and __message will all be retrieved without entering that function, and I don't think __file__ needs to get a normal AttributeError instead of ModuleNotFoundError.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (21e5d04) to head (2f99692).
Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##             main      #153       +/-   ##
============================================
+ Coverage   87.27%   100.00%   +12.72%     
============================================
  Files           1         1               
  Lines         110       108        -2     
  Branches       20        19        -1     
============================================
+ Hits           96       108       +12     
+ Misses          7         0        -7     
+ Partials        7         0        -7     

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[stefanv]

Thanks @effigies!