[Specs] Fix for reinitialization of constant.

The actual warning:
./spec/unit/sandbox_spec.rb:4: warning: already initialized constant TMP_POD_ROOT
Pod::Sandbox

spec/unit/sandbox_spec.rb

 require File.expand_path('../../spec_helper', __FILE__)
 require 'tmpdir'
 
-TMP_POD_ROOT = ROOT + "tmp" + "podroot"
+TMP_POD_ROOT = ROOT + "tmp" + "podroot" unless defined? TMP_POD_ROOT
 
 describe Pod::Sandbox do