Add JET tests

[JoshuaLampert]

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

As discussed in #501 (comment), this adds JET.jl tests. I'll try to also fix any potential issues reported by JET.jl here.

[JoshuaLampert]

So the remaining errors from JET.jl are all because of StaticArraysCore.similar_type. The problem here is that RecursiveArrayTools.jl depends only on StaticArraysCore.jl, where similar_type is defined, but only as a stub. The actual implementation lives in StaticArrays.jl, which is not visible to RecursiveArrayTools.jl. I'm not sure what the best option to fix this is. Any ideas, @ChrisRackauckas?
Of course, this is usually not a problem because we have StaticArrays.jl loaded when we want to call this method, but I understand why JET.jl is complaining.

[ChrisRackauckas]

You can set it to ignore that function