[ruby/irb] Fix test warnings (https://github.com/ruby/irb/pull/698)

* Encoding should be saved before creating Irb objects

* Fix unused local warning

https://github.com/ruby/irb/commit/036ec31034

test/irb/test_context.rb

@@ -93,7 +93,7 @@ module TestIRB
     def test_prompt_n_deprecation
       irb = IRB::Irb.new(IRB::WorkSpace.new(Object.new))
 
-      out, err = capture_output do
+      _, err = capture_output do
         irb.context.prompt_n = "foo"
         irb.context.prompt_n
       end

test/irb/test_irb.rb

@@ -94,8 +94,8 @@ module TestIRB
     end
 
     def setup
-      @irb = build_irb
       save_encodings
+      @irb = build_irb
     end
 
     def teardown