add integration test for Environment#run_builder

build_tests/simple/run_builder.rb

@@ -0,0 +1,12 @@
+class MyObject < Rscons::Builder
+  def run(options)
+    target, sources, cache, env, vars = options.values_at(:target, :sources, :cache, :env, :vars)
+    env.run_builder(env.builders["Object"], target, sources, cache, vars)
+  end
+end
+
+Rscons::Environment.new do |env|
+  env.add_builder(MyObject.new)
+  env.MyObject("simple.o", "simple.c")
+  env.Program("simple.exe", "simple.o")
+end