MAINT improve isolation for micropip tests. I added a new `wheel_base` fixture which creates a new temporary directory and mocks `site.getsitepackages()` to return it. I also removed the `WHEEL_BASE` global variable from micropip and instead calculate it each time in `micropip.install`.
from pyodide_test_runner import run_in_pyodide


@run_in_pyodide(packages=["Jinja2"])
def test_jinja2(selenium):
    import jinja2

    template = jinja2.Template("Hello {{ name }}!")
    content = template.render(name="Zach")
    assert content == "Hello Zach!"

from pyodide_test_runner import run_in_pyodide


@run_in_pyodide(packages=["Jinja2"])
def test_jinja2(selenium):
    import jinja2

    template = jinja2.Template("Hello {{ name }}!")
    content = template.render(name="Zach")
    assert content == "Hello Zach!"
