"Embed" test_bdist_wheel files and execution

[abravalheri]

Summary of changes

Currently test_bdist_wheel:

• Relies on external files placed on the setuptools/tests/bdist_wheel_testdata:
  • The problem with this approach is that it complicates the management of MANIFEST.in and is error prone (see [BUG] Test data missing (extension.c files and more) missing in 70.1.0 sdist #4428).
    Some files are missing in the sdist and others may be erroneously added depending on how clean your directory is (e.g. setuptools/tests/bdist_wheel_testdata/*/build).
    This affects setuptools a lot because we cannot use setuptools-scm, but even if we could it also complicates "git" management itself...
    I found a couple of files that should not have been committed (e.g. .so files that were generated by tests).
• Invokes python setup.py bdist_wheel via subprocess:
  • The problem with this approach (other than using the deprecated CLI of setup.py) is that it complicates collecting coverage data.

This PR "embeds" both of these aspects:

• all required files are generated during the tests runtime on temporary directories
• the bdist_wheel command runs via API in the current process.

Closes #4428

Pull Request Checklist

• Changes have tests
• News fragment added in newsfragments/.
  (See documentation for details)