update build_tests_spec to invoke rscons in a subprocess for each test

build_tests/json_to_yaml/Rsconsfile

@@ -0,0 +1,15 @@
+Rscons::Environment.new do |env|
+  require 'json'
+  require 'yaml'
+  env.add_builder(:JsonToYaml) do |target, sources, cache, env, vars|
+    unless cache.up_to_date?(target, :JsonToYaml, sources, env)
+      cache.mkdir_p(File.dirname(target))
+      File.open(target, 'w') do |f|
+        f.write(YAML.dump(JSON.load(IO.read(sources.first))))
+      end
+      cache.register_build(target, :JsonToYaml, sources, env)
+    end
+    target
+  end
+  env.JsonToYaml('foo.yml', 'foo.json')
+end