Since version v20.23.0, `virtualenv` will no longer include `wheel` and
`setuptools` in the created folders.

Some tests in the setuptools test suite assume that these packages are
always present. So we need to adequate these tests.

This enhances environment isolation, as in special cases, like downstream
distro packaging, PYTHONPATH can be set to point to a specific setuptools
codebase. When it leaks, it shadows the virtual environment's paths and
produces wrong test results.

Instead of re-building/installing setuptools from the source tree
every time, the tests now rely on the venv, wheel and sdist fixtures
(the venv fixture is populated from sdist/wheel).

Moreover migrate `test_virtualenv` to use `jaraco.envs`
(so it uses the same libraries ad `test_distutils_adoption`).

… and change it to install the pre-build setuptools wheel (fixture)
instead of installing from the source tree

They should be build once for each session and be able to be re-used in
parallel (assuming read-only) for all tests.

This is useful when dealing with virtual environments