Undo removal of test docstring

It was very hard for me to figure out what this test was designed
to test. It turns out that when this test was promoted from unit
to functional someone forgot to copy the docstring

9faa9aef29

tests/functional/test_download.py

@@ -850,6 +850,9 @@ def test_download_file_url_existing_bad_download(
 def test_download_http_url_bad_hash(
     shared_script, shared_data, tmpdir, mock_server
 ):
+    """
+    If already-downloaded file has bad checksum, re-download.
+    """
     download_dir = tmpdir / 'download'
     download_dir.mkdir()
     downloaded_path = download_dir / 'simple-1.0.tar.gz'