Replace tests using mock_fetch with selenium-based tests

[ryanking13]

Tests with mock_fetch (and mock_importlib ) is quite fragile and hard to maintain. I think we should replace those with selenium-based tests as much as possible.

When micropip was in pyodide/pyodide, we avoided using selenium tests because selenium-based tests would increase the overall test time. But now, micropip lives in a separate repository. So, I think taking a few more minutes to run the test is not a big concern.