Add MA tests back

Author: blegat

test/commutativetests.jl

@@ -2,8 +2,5 @@ for file in readdir(joinpath(@__DIR__, "commutative"))
     if file == "complex.jl" && !isdefined(Mod, Symbol("@complex_polyvar"))
         continue
     end
-    if file == "mutable_arithmetics.jl"
-        continue
-    end
     include(joinpath(@__DIR__, "commutative", file))
 end