move struct defn in tests to top level

Author: oxinabox

test/rulesets/Base/base.jl

@@ -1,3 +1,7 @@
+mutable struct MDemo
+    x::Float64
+end
+
 @testset "base.jl" begin
     @testset "zero/one" begin
         for f in [zero, one]
@@ -20,10 +24,6 @@
     end
 
     @testset "setfield!" begin
-        mutable struct MDemo
-            x::Float64
-        end
-
         test_frule(setfield!, MDemo(3.5) ⊢ MutableTangent{MDemo}(; x=2.0), :x, 5.0)
         test_frule(setfield!, MDemo(3.5) ⊢ MutableTangent{MDemo}(; x=2.0), 1, 5.0)
     end