Update tests and fix use_support support

Author: YingboMa

src/Mechanical/TranslationalPosition/sources.jl

@@ -5,8 +5,12 @@ Input signal acting as external force on a flange
 """
 @mtkmodel Force begin
     @parameters begin
+        use_support = false
         s = 0
     end
+    @extend (flange,) = partial_element = PartialElementaryOneFlangeAndSupport2(;
+        use_support = use_support isa Bool ? use_support :
+                      ModelingToolkit.getdefault(use_support))
     @components begin
         flange = Flange(; s = s)
         f = RealInput()

test/Mechanical/multibody.jl

@@ -23,6 +23,7 @@ eqs = [connect(link1.TX1, cart.flange) #, force.flange)
 @named model = ODESystem(eqs, t, [], []; systems = [link1, link2, cart, fixed])
 
 sys = structural_simplify(model)
+@test length(states(sys)) == 6
 
 # The below code does work...
 #=