Check the behavior of micropip.freeze with pyodide-lock

[ryanking13]

micropip.freeze is now used in pyodide-pack (https://github.com/pyodide/pyodide-pack/pull/31), but I think we don't have strong enough test cases for micropip.freeze.

So I think it would be good to add some tests that validate the output of micropip.freeze with pyodide-lock.