Implement `` condition keyword (shell cmd status)

lib/producer/core/condition/dsl.rb

@@ -13,6 +13,7 @@ module Producer
           end
         end
 
+        define_test :`,               Tests::ShellCommandStatus
         define_test :file_contains,   Tests::FileContains
         define_test :has_env,         Tests::HasEnv
         define_test :has_executable,  Tests::HasExecutable

lib/producer/core/tests/shell_command_status.rb

@@ -0,0 +1,18 @@
+module Producer
+  module Core
+    module Tests
+      class ShellCommandStatus < Test
+        def verify
+          remote.execute(command)
+          true
+        rescue RemoteCommandExecutionError
+          false
+        end
+
+        def command
+          arguments.first
+        end
+      end
+    end
+  end
+end