Add tests capturing expected distutils behavior. Ref #417.

setuptools/tests/test_distutils_adoption.py

+import os
+import pytest
+
+
+@pytest.fixture
+def env(virtualenv):
+    virtualenv.run(['pip', 'uninstall', '-y', 'setuptools'])
+    virtualenv.run(['pip', 'install', os.getcwd()])
+    return virtualenv
+
+
+def find_distutils(env, imports='distutils'):
+    py_cmd = 'import {imports}; print(distutils.__file__)'.format(**locals())
+    cmd = ['python', '-c', py_cmd]
+    return env.run(cmd, capture=True, text=True)
+
+
+def test_distutils_stdlib(env):
+    """
+    Ensure stdlib distutils is used when appropriate.
+    """
+    assert '.env' not in find_distutils(env).split(os.sep)
+
+
+def test_distutils_local_with_setuptools(env):
+    """
+    Ensure local distutils is used when appropriate.
+    """
+    env.env.update(SETUPTOOLS_USE_DISTUTILS='local')
+    loc = find_distutils(env, imports='setuptools, distutils')
+    assert '.env' in loc.split(os.sep)
+
+
+@pytest.mark.xfail(reason="#2259")
+def test_distutils_local(env):
+    env.env.update(SETUPTOOLS_USE_DISTUTILS='local')
+    assert '.env' in find_distutils(env).split(os.sep)