Fix test warnings (#698)

* Encoding should be saved before creating Irb objects

* Fix unused local warning

Author: st0012

test/irb/test_context.rb

@@ -93,7 +93,7 @@ def test_eval_input_raise2x
     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 @@ def dynamic_prompt(&block)
     end
 
     def setup
-      @irb = build_irb
       save_encodings
+      @irb = build_irb
     end
 
     def teardown