Fix X display handling in rspec test suite

* Hide any available X display from test suite context;
* Ensure the manager never connect or block.

spec/uh/wm/cli_spec.rb

@@ -15,8 +15,7 @@ module Uh
           described_class.run arguments, stdout: stdout, stderr: stderr
         end
 
-        # FIXME: remove this hack we currently need to prevent the Runner from
-        # blocking.
+        # Prevent Runner from connecting a Manager and blocking.
         before { allow(Runner).to receive :run }
 
         it 'builds a new CLI with given arguments' do

spec/uh/wm/manager_spec.rb

@@ -14,6 +14,9 @@ module Uh
       describe '#connect' do
         let(:block) { proc { } }
 
+        # Prevent Manager to open a real display.
+        before { allow(display).to receive :open }
+
         it 'opens the display' do
           expect(manager.display).to receive :open
           manager.connect