Resetting package.loaded after a test in Lua

I am developing unit tests in Lua and a major part of these tests is changing package.loaded. This goes from

a = require "parser"
a.b = nil

to

package.loaded["checker"] = function() return true end

And the situation where I modify an entry of package.loaded that was loaded before the test started.

I would like to reset package.loaded after every test. But I have a hard time. I tried wrapping require, which solves some problems, but does not solve the problem of the second and the third example. How can I properly make a savepoint of package.loaded right before the tests and reload that savepoint just before I start a new test? Or just roll back package.loaded to the one just after the interpeter started?

This is actually the answer of @EgorSkriptunoff, but he placed it as a comment.

For me, it was good enough to just make a shallow-copy of bot the package.loaded and the _G. Shallow-copy can be found here: http://lua-users.org/wiki/CopyTable:

function shallowcopy(orig)
    local orig_type = type(orig)
    local copy
    if orig_type == 'table' then
        copy = {}
        for orig_key, orig_value in pairs(orig) do
            copy[orig_key] = orig_value
        end
    else -- number, string, boolean, etc
        copy = orig
    end
    return copy
end

The actual testing then looks like this:

local packageLoadedCopy = shallowcopy(package.loaded)
local gCopy = shallowcopy(_G)
for _, test in ipairs(tests) do
    -- Do the actual test
    -- Clean up the environment
    local markDeletion = {}
    local markModify = {}
    for name in pairs(package.loaded) do
        if not packageLoadedCopy[name] then
            table.insert(markDeletion, name)
        elseif packageLoadedCopy[name] ~= package.loaded[name] then
            table.insert(markModify, name)
        end
    end
    for _, name in pairs(markDeletion) do
        package.loaded[name] = nil
    end
    for _, name in pairs(markModify) do
        package.loaded[name] = packageLoadedCopy[name]
    end
    markDeletion, markModify = {}, {}
    for name in pairs(_G) do
        if not gCopy[name] then
            table.insert(markDeletion, name)
        elseif _G[name] ~= gCopy[name] then
            table.insert(markModify, name)
        end
    end
    for _, name in pairs(markDeletion) do
        _G[name] = nil
    end
    for _, name in pairs(markModify) do
        _G[name] = gCopy[name]
    end
end

Do note that simply

package.loaded = packageLoadedCopy
_G = gCopy

Does not work, and I am not sure why.