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=["xarray"])
def test_xarray(selenium):
    import xarray as xr

    da = xr.DataArray(data=[[0, 1, 2], [3, 4, 5]], dims=("x", "y"))
    xr.testing.assert_equal(a=da.mean(), b=xr.DataArray(data=2.5))

from pyodide_test_runner import run_in_pyodide


@run_in_pyodide(packages=["xarray"])
def test_xarray(selenium):
    import xarray as xr

    da = xr.DataArray(data=[[0, 1, 2], [3, 4, 5]], dims=("x", "y"))
    xr.testing.assert_equal(a=da.mean(), b=xr.DataArray(data=2.5))
