luajit: add test

Formula/luajit.rb

@@ -28,4 +28,12 @@ class Luajit < Formula
 
     system 'make', 'amalg', 'install', *args
   end
+
+  test do
+    system "#{bin}/luajit", "-e", <<-EOS.strip
+      local ffi = require("ffi")
+      ffi.cdef("int printf(const char *fmt, ...);")
+      ffi.C.printf("Hello %s!\\n", "#{ENV['USER']}")
+      EOS
+  end
 end