spec/pry_defaults: silence unwanted deprecation warnings

lib/pry/pry_instance.rb

@@ -478,7 +478,7 @@ class Pry
 
       unless @prompt_warn
         @prompt_warn = true
-        warn(
+        Kernel.warn(
           "warning: setting prompt with help of " \
           "`Pry.config.prompt = [proc {}, proc {}]` is deprecated. " \
           "Use Pry::Prompt API instead"

spec/pry_defaults_spec.rb

@@ -13,6 +13,9 @@ describe "test Pry defaults" do
   end
 
   describe "input" do
+    # Silence deprecation warnings.
+    before { allow(Kernel).to receive(:warn) }
+
     it 'should set the input default, and the default should be overridable' do
       Pry.config.input = InputTester.new("5")
       Pry.config.output = @str_output