Add Barrier build test

build_tests/simple/barrier_builder.rb

@@ -0,0 +1,15 @@
+class B < Builder
+  def run(*args)
+    puts "B:#{@target}"
+    true
+  end
+end
+
+env do |env|
+  env.add_builder(B)
+  env.B("one")
+  env.B("two")
+  env.B("three")
+  env.Barrier(:bar, %w[two three])
+  env.depends("one", :bar)
+end