Add TEST_ENVIRONMENT option and set PYTHONSAFEPATH=1 in the test env (#2388)

* Add TEST_ENVIRONMENT option and set PYTHONSAFEPATH=1 in the test env

Closes https://github.com/pypa/cibuildwheel/issues/2358

* Add toml docs examples

* Add TOML examples

* Use double-quotes for windows

test/test_testing.py

@@ -66,6 +66,12 @@ class TestSpam(TestCase):
         bits = struct.calcsize("P") * 8
         if bits == 32:
             self.assertIn(platform.machine(), ["i686", "armv7l","armv8l", "wasm32"])
+
+    def test_pythonsafepath(self):
+        # check that the PYTHONSAFEPATH environment variable is set
+        value = os.environ.get("PYTHONSAFEPATH")
+        self.assertIsNotNone(value)
+        self.assertNotEqual(value, "")
 '''
 
 
@@ -229,3 +235,19 @@ def test_test_sources(tmp_path):
     # also check that we got the right wheels
     expected_wheels = utils.expected_wheels("spam", "0.1.0")
     assert set(actual_wheels) == set(expected_wheels)
+
+
+def test_test_environment(tmp_path):
+    project_dir = tmp_path / "project"
+    test_projects.new_c_project().generate(project_dir)
+
+    actual_wheels = utils.cibuildwheel_run(
+        project_dir,
+        add_env={
+            "CIBW_TEST_ENVIRONMENT": "MYVAR=somevalue PYTHONSAFEPATH=",
+            "CIBW_TEST_COMMAND": "python -c \"import os; assert os.environ.get('MYVAR') == 'somevalue'; assert os.environ.get('PYTHONSAFEPATH') == ''\"",
+        },
+    )
+    # also check that we got the right wheels
+    expected_wheels = utils.expected_wheels("spam", "0.1.0")
+    assert set(actual_wheels) == set(expected_wheels)