local moduleFoo={}
local moduleBaz=  require("moduleBaz")

moduleFoo.doSomething = function (arg)

  if moduleBaz.bar.neatMethod(arg) then
     --does something interesting
  end

end

return moduleFoo

local moduleBaz={}
moduleBaz.bar= {}

moduleBaz.bar.neatMethod=function(arg)
   --does something neat
end
return moduleBaz

package.loaded.moduleBaz= nil
    local moduleBaz = {}
    moduleBaz.bar = {}
    moduleBaz.bar.neatMethod= function(guid) return true end

    package.preload['moduleBaz'] = function ()
        return moduleBaz
    end

   local foo= require("moduleFoo")
   foo.doSomething('asdasdasda')--real moduleBaz is called, not my mock!

return moduleBaz

package.loaded.moduleBaz = {
  bar = {
    neatmethod = function(arg)
      -- your mock code here
    end,
  }
}

local moduleBaz={}
moduleBaz.bar= {}
moduleBaz.bar.neatMethod=function(arg)
  print "baz"
  return true
end
return moduleBaz

local moduleFoo={}
local moduleBaz=  require("moduleBaz")
  moduleFoo.doSomething = function (arg)
  if moduleBaz.bar.neatMethod(arg) then
    print "foo"
  end
end
return moduleFoo

package.loaded.moduleBaz= nil
local moduleBaz = {}
moduleBaz.bar = {}
moduleBaz.bar.neatMethod= function(guid) print "mock" return true end

package.preload['moduleBaz'] = function ()
    return moduleBaz
end

local foo= require("moduleFoo")
foo.doSomething('asdasdasda')--real moduleBaz is called, not my mock!