Add test case for proper build isolation

If there's not a proper build isolation then,
the wheel for `cp27mu` will pick up the `spam.so` built
for `cp27m` resulting in an import error.

test/test_unicode.py

@@ -0,0 +1,31 @@
+import textwrap
+
+import pytest
+
+from . import utils
+from . import test_projects
+
+project_with_unicode = test_projects.new_c_project(
+    spam_c_function_add=textwrap.dedent(r'''
+        {
+            Py_XDECREF(PyUnicode_FromStringAndSize("foo", 4));
+        }
+    '''),
+)
+
+
+def test(tmp_path):
+    if utils.platform != 'linux':
+        pytest.skip('the docker test is only relevant to the linux build')
+
+    project_dir = tmp_path / 'project'
+    project_with_unicode.generate(project_dir)
+
+    # build the wheels
+    actual_wheels = utils.cibuildwheel_run(project_dir, add_env={
+        'CIBW_TEST_COMMAND': 'python -c "import spam"'
+    })
+
+    # check that the expected wheels are produced
+    expected_wheels = utils.expected_wheels('spam', '0.1.0')
+    assert set(actual_wheels) == set(expected_wheels)