Implement `file_contains' condition keyword

lib/producer/core/condition/dsl.rb

@@ -13,9 +13,10 @@ module Producer
           end
         end
 
-        define_test :has_env,   Tests::HasEnv
-        define_test :has_dir,   Tests::HasDir
-        define_test :has_file,  Tests::HasFile
+        define_test :file_contains, Tests::FileContains
+        define_test :has_env,       Tests::HasEnv
+        define_test :has_dir,       Tests::HasDir
+        define_test :has_file,      Tests::HasFile
 
         attr_reader :block, :env, :tests
 

lib/producer/core/tests/file_contains.rb

@@ -0,0 +1,18 @@
+module Producer
+  module Core
+    module Tests
+      class FileContains < Test
+        def verify
+          content = file_content
+          content ? content.include?(arguments[1]) : false
+        end
+
+        private
+
+        def file_content
+          fs.file_read(arguments[0])
+        end
+      end
+    end
+  end
+end