test: exercise SISO initialization through ODEProblem (#491)

The standalone initialization problem represents an intermediate system, so it can lose defaults or observed variables from the original ODE system. Construct the ODE problem and assert the same initialized values through its public indexing API.

Co-authored-by: ChrisRackauckas-Claude <accounts@chrisrackauckas.com>

Author: ChrisRackauckas-Claude

test/utils.jl

@@ -1,7 +1,7 @@
 using ModelingToolkitStandardLibrary.Blocks
 using SciCompDSL
 using ModelingToolkit
-using OrdinaryDiffEq
+using SciMLBase: ODEProblem
 using ModelingToolkit: t_nounits as t, D_nounits as D
 using Symbolics
 
@@ -34,11 +34,10 @@ end
     @named iosys = System(connect(c.output, so.input), t, systems = [so, c])
     sys = mtkcompile(iosys)
 
-    initprob = ModelingToolkit.InitializationProblem(sys, 0.0)
-    initsol = solve(initprob)
+    prob = ODEProblem(sys, Dict{Any, Any}(), (0.0, 1.0))
 
-    @test initsol[sys.so.xd] == 1.0
-    @test initsol[sys.so.u] == 1.0
+    @test prob[sys.so.xd] == 1.0
+    @test prob[sys.so.u] == 1.0
 end
 
 @test_deprecated RealInput(; name = :a, u_start = 1.0)