add non-broken test

test/test_JointUSR_RRR.jl

@@ -18,8 +18,8 @@ W(args...; kwargs...) = Multibody.world
         world = W()
         j1 = JointUSR(positive_branch=true, use_arrays=false)
         fixed = FixedTranslation(r=[1,0,0])
-        b1 = Body(isroot=false, neg_w=true)
-        p1 = Prismatic(state_priority=100)
+        b1 = Body(m=1, isroot=false, neg_w=true)
+        p1 = Prismatic(state_priority=100, n = [1, 0, 0])
     end
     @equations begin
         connect(world.frame_b, j1.frame_a, fixed.frame_a)
@@ -32,11 +32,13 @@ end
 @named model = TestUSR()
 model = complete(model)
 ssys = structural_simplify(IRSystem(model))
+@test length(unknowns(ssys)) == 2
 ##
 
-prob = ODEProblem(ssys, [model.b1.a_0[1]=>0.0, D(D(model.p1.s))=>0.0], (0.0, 1.0))
+prob = ODEProblem(ssys, [model.p1.v => 0.0], (0.0, 1.4))
 sol = solve(prob, FBDF(autodiff=true))
-@test_broken !all(iszero, sol(1.0))
+@test sol(1.0, idxs=model.p1.s) ≈ -2.8 rtol=0.01 # test vs. OpenModelica
+
 
 # NOTE: I was working on trying to register the compute_angle function so that there are no symbolic arguments left in the generated code