Add tests

[jakkn]

build.rb needs test coverage.

[jakkn]

Challenging due to build.rb manipulating the file system. FakeFS seemed at first to be the right approach, but the scope of FakeFS is lost for system calls because FakeFS only replaces File and FileUtils for the running ruby thread. As long as Rake commands are issued along the lines of system "rake", "--rakefile", "pack.rake" this approach will not work.