test: fix CSE hack test

Author: AayushSabharwal

test/structural_transformation/utils.jl

@@ -92,7 +92,7 @@ end
         @mtkbuild sys = ODESystem(
             [D(y) ~ foo(x), D(x) ~ sum(y), zeros(2) ~ foo(prod(z))], t)
         @test length(equations(sys)) == 5
-        @test length(observed(sys)) == 2
+        @test length(observed(sys)) == 4
         prob = ODEProblem(
             sys, [y => ones(2), z => 2ones(2), x => 3.0], (0.0, 1.0), [foo => _tmp_fn2])
         @test_nowarn prob.f(prob.u0, prob.p, 0.0)