Fail when another window manager is already running

features/support/env.rb

@@ -14,3 +14,10 @@ Headless.new.start
 After do |scenario|
   @process and @process.terminate
 end
+
+Around '@other_wm_running' do |scenario, block|
+  @other_wm = ChildProcess.build('twm')
+  @other_wm.start
+  block.call
+  @other_wm.stop
+end