Avoid logged messages showing up as part of our spec suite output.

Before this change, `rspec --seed 31448` would result in some extra
messages showing up as part of the output of our spec suite:

> RSpec is shutting down and will print the summary report... Interrupt again to force quit.

This fixes the issue, expecting the output instead.

lib/rspec/core/runner.rb

@@ -175,7 +175,7 @@ def self.handle_interrupt
           exit!(1)
         else
           RSpec.world.wants_to_quit = true
-          STDERR.puts "\nRSpec is shutting down and will print the summary report... Interrupt again to force quit."
+          $stderr.puts "\nRSpec is shutting down and will print the summary report... Interrupt again to force quit."
         end
       end
     end

spec/rspec/core/runner_spec.rb

@@ -137,19 +137,20 @@ def interrupt
           interrupt
         end
 
-        it "does not exit immediately" do
+        it "does not exit immediately, but notifies the user" do
           Runner.send(:trap_interrupt)
           expect(Runner).not_to receive(:exit)
           expect(Runner).not_to receive(:exit!)
-          interrupt
+
+          expect { interrupt }.to output(/RSpec is shutting down/).to_stderr
         end
       end
 
       context "with SIGINT twice" do
         it "exits immediately" do
           Runner.send(:trap_interrupt)
           expect(Runner).to receive(:exit!).with(1)
-          interrupt
+          expect { interrupt }.to output(//).to_stderr
           interrupt
         end
       end