add unit tests for State class

Rsconscript

@@ -3,8 +3,13 @@ configure do
 end
 
 build do
-  Environment.new do |env|
+  main_env = Environment.new do |env|
     env["D_IMPORT_PATH"] << "src"
-    env.Program("peg-puzzle", glob("src/**/*.d"))
+    env["sources"] = glob("src/**/*.d")
+    env.Program("peg-puzzle", "${sources}")
+  end
+  test_env = main_env.clone do |env|
+    env["DFLAGS"] += %w[-g -funittest]
+    env.Program("peg-puzzle-test", "${sources}")
   end
 end