Force Display test double usage in Runner.run specs

We need to mock Display.new to return a test double with #open
disabled, in order to prevent the manager to initiate real connections
to current display and because we don't want to inject a Display
instance all the way down from Runner to Manager (at least not yet).

spec/uh/wm/runner_spec.rb

@@ -7,6 +7,11 @@ module Uh
       describe '.run' do
         subject(:run) { described_class.run env, stopped: true }
 
+        before do
+          allow(Display)
+            .to receive(:new) { double('display', open: nil).as_null_object }
+        end
+
         it 'builds a new Runner with given env' do
           expect(described_class)
             .to receive(:new).with(env, anything).and_call_original