ci: run tests in cibuildwheel

Previously we weren't running tests against the built wheel. The wheel
build could be different from the build used in the tests workflow.

Let's test our wheels to add extra confidence about their integrity.

Author: indygreg

pyproject.toml

@@ -65,9 +65,25 @@ build-frontend = "build[uv]"
 # Turn compiler warnings into errors.
 environment = "ZSTD_WARNINGS_AS_ERRORS=1"
 
+test-command = [
+    "pytest -v --numprocesses=auto",
+    "PYTHON_ZSTANDARD_IMPORT_POLICY=cffi pytest -v --numprocesses=auto",
+]
+test-groups = ["dev"]
+test-requires = ["cffi"]
+test-sources = ["tests"]
+
 [tool.cibuildwheel.linux]
 before-all = "if [ -x /usr/bin/yum ]; then yum install -y libffi-devel; fi"
 
+[tool.cibuildwheel.windows]
+# Environment variable syntax is different on Windows.
+test-command = [
+    "pytest -v --numprocesses=auto",
+    "set PYTHON_ZSTANDARD_IMPORT_POLICY=cffi",
+    "pytest -v --numprocesses=auto",
+]
+
 # This is needed to suppress generation of license-file, which isn't
 # compatible with metadata version 2.1, which our pinned setuptools
 # version currently uses.