Add verification of linked scripts on *nix

Runs a bunch of `/usr/bin/foo -v` type commands. This tests a few things:

* The post install script created the symlinks
* The symlink targets (executables in `/opb/chefdk/bin` exist)
* The executables work correctly when symlinked from outside `/opt/chefdk/bin`
* `knife` loads subcommands without blowing up.

@opscode/client-engineers