replace mutable struct with NamedTuple in test

Author: penelopeysm

test/submodels.jl

@@ -106,18 +106,14 @@ using Test
         end
 
         @testset "Complex prefixes" begin
-            mutable struct P
-                a::Float64
-                b::Float64
-            end
             @model function f()
                 x = Vector{Float64}(undef, 1)
                 x[1] ~ Normal()
                 y ~ Normal()
                 return x[1]
             end
             @model function g()
-                p = P(1.0, 2.0)
+                p = (; a=1.0, b=2.0)
                 p.a ~ to_submodel(f())
                 p.b ~ Normal()
                 return (p.a, p.b)