adept integration test to changes in command_wrapper

command_wrapper tries to verify the connection (which includes sudo)
using a call to "echo". If this fails, it'll raise an error. This change
is now reflected in the test.

fixes #34.

test/integration/sudo/passwd.rb

@@ -10,8 +10,8 @@
   end
 
   it 'is not running sudo without password' do
-    run_as('whoami', { sudo: true })
-      .exit_status.wont_equal 0
+    err = ->{Train.create('local', { sudo: true }).connection}.must_raise Train::UserError
+    err.message.must_match /Sudo requires a password/
   end
 
   it 'is running passwd sudo' do