Add a test that the runner script works in environments without pip

This ensures that the runner script can be used in environments where
pip is not installed.

tests/functional/test_pip_runner_script.py

@@ -0,0 +1,20 @@
+import os
+from pathlib import Path
+
+from pip import __version__
+from tests.lib import PipTestEnvironment
+
+
+def test_runner_work_in_environments_with_no_pip(
+    script: PipTestEnvironment, pip_src: Path
+) -> None:
+    runner = pip_src / "src" / "pip" / "__pip-runner__.py"
+
+    # Ensure there's no pip installed in the environment
+    script.pip("uninstall", "pip", "--yes", use_module=True)
+    script.pip("--version", expect_error=True)
+
+    # The runner script should still invoke a usable pip
+    result = script.run("python", os.fspath(runner), "--version")
+
+    assert __version__ in result.stdout