move mutable struct definition outside of loop

[penelopeysm]

WARNING: Method definition (::Type{DPPLSubmodelTests.P})(Float64, Float64) in module DPPLSubmodelTests at /home/runner/work/DynamicPPL.jl/DynamicPPL.jl/test/submodels.jl:110 overwritten on the same line (check for duplicate calls to `include`).
WARNING: Method definition (::Type{DPPLSubmodelTests.P})(Any, Any) in module DPPLSubmodelTests at /home/runner/work/DynamicPPL.jl/DynamicPPL.jl/test/submodels.jl:110 overwritten on the same line (check for duplicate calls to `include`).

It looks like this

DynamicPPL.jl/test/submodels.jl

Lines 109 to 112 in 3e54c2d

	mutable struct P
	a::Float64
	b::Float64
	end

is being defined twice because it's inside a nested testset, so we should move it out to just silence that warning.