test, docs: fix tests and docs for MTKv9

[AayushSabharwal]

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

Attention: Patch coverage is 50.00000% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 37.54%. Comparing base (0998e07) to head (e5e1172).

Files	Patch %	Lines
src/scimlfunctions.jl	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #634      +/-   ##
==========================================
+ Coverage   28.86%   37.54%   +8.67%     
==========================================
  Files          55       55              
  Lines        4285     4294       +9     
==========================================
+ Hits         1237     1612     +375     
+ Misses       3048     2682     -366     

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[AayushSabharwal]

There are more fixes than it looks like there are, I'm working on this locally. MTK needs a couple changes to support indexing with Symbols (prob.ps[:a] to refer to @parameters a)

[ChrisRackauckas]

Rebase this. I fixed some test cases

[AayushSabharwal]

Requires the fix in SciML/ModelingToolkit.jl#2524