test: use frame kwarg for tests SkyModels

[steven-murray]

Description

Simply updates the SkyModel() call in tests to use the frame keyword.

Issues

Checklist

I have

• Added a test covering your new feature adequately?
• Added a docstring (or note to a docstring) describing your feature?
• (Optional): Added a tutorial / section to a tutorial showing usage of your new feature?
• Important: this feature does not break API compatibility.

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (d2e21d5) 100.00% compared to head (78dccaa) 100.00%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #67   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            8         8           
  Lines          567       567           
  Branches        88        88           
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  Hits           567       567           

Flag	Coverage Δ
unittests	99.29% <ø> (ø)

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[r-pascua]

Looks good to me! Thanks!