Avoid including independent twice (#321)

Author: blegat

test/commutativetests.jl

@@ -1,4 +1,8 @@
 for file in readdir(joinpath(@__DIR__, "commutative"))
+    if file == "independent.jl"
+        # It's included by `gcd.jl`
+        continue
+    end
     if file == "complex.jl" && !isdefined(Mod, Symbol("@complex_polyvar"))
         continue
     end