Add back accidentally removed tests

Author: YingboMa

test/sdesystem.jl

@@ -584,3 +584,33 @@ end
     @test μ≈μmod atol=2σ
     @test σ > σmod
 end
+
+@variables t
+D = Differential(t)
+sts = @variables x(t) y(t) z(t)
+ps = @parameters σ ρ
+@brownian β η
+s = 0.001
+β *= s
+η *= s
+
+eqs = [D(x) ~ σ * (y - x) + x * β,
+    D(y) ~ x * (ρ - z) - y + y * β + x * η,
+    D(z) ~ x * y - β * z + (x * z) * β]
+@named sys1 = System(eqs, t)
+sys1 = structural_simplify(sys1)
+
+drift_eqs = [D(x) ~ σ * (y - x),
+    D(y) ~ x * (ρ - z) - y,
+    D(z) ~ x * y]
+
+diffusion_eqs = [s*x 0
+                 s*y s*x
+                 (s * x * z)-s * z 0]
+
+sys2 = SDESystem(drift_eqs, diffusion_eqs, t, sts, ps, name = :sys1)
+@test sys1 == sys2
+
+prob = SDEProblem(sys1, sts .=> [1.0, 0.0, 0.0],
+                  (0.0, 100.0), ps .=> (10.0, 26.0))
+solve(prob, LambaEulerHeun(), seed = 1)