Be more informative about test-data provenance

test-data/to-header-dot-as-name.mbox

@@ -5,4 +5,4 @@ Date: Mon, 21 Dec 2020 12:00:00 -0000
 Message-ID: <87ab5rvds7.fsf@example.com>
 Subject: This is the Subject header.
 
-Here is the body of the message.
+The original message was ~/mail/creative-commons/1.gz FWIW.