Simplify spec and make it consistent with the one below

Author: eregon

spec/ruby/core/struct/shared/inspect.rb

@@ -9,12 +9,12 @@
   end
 
   it "returns a string representation without the class name for structs nested in anonymous classes" do
-    obj = Object.new
-    obj.singleton_class.class_eval <<~DOC
+    c = Class.new
+    c.class_eval <<~DOC
       class Foo < Struct.new(:a); end
     DOC
 
-    obj.singleton_class::Foo.new("").send(@method).should == '#<struct a="">'
+    c::Foo.new("").send(@method).should == '#<struct a="">'
   end
 
   it "returns a string representation without the class name for structs nested in anonymous modules" do