Use assert shutil.which("example") for testing entrypoint script existence

Imported from gh:painless-software/python-cli-test-helpers#52

Currently, the examples demonstrate using the package's shell command to verify the existence of an entrypoint script.

Another way to perform the verification would be shutil.which, which is a platform-independent implementation of where (Windows) and which (Unix/Linux/macOS). Example:

def test_entrypoint():
    assert shutil.which("example")