ci: run SII testset in RAT downstream tests

[AayushSabharwal]

Close #31

Requires SciML/RecursiveArrayTools.jl#362

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

Add any other context about the problem here.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 35.91%. Comparing base (a7c70c8) to head (75aa870).
Report is 10 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #54      +/-   ##
==========================================
- Coverage   38.83%   35.91%   -2.93%     
==========================================
  Files           7        7              
  Lines         291      401     +110     
==========================================
+ Hits          113      144      +31     
- Misses        178      257      +79     

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