Add reproduction data for the backslash problem

This input data produces problematic output like this:

  "Firstname2\ \\"Lastname2 <second_recipient@receivers.example.com>"

Not sure yet whether the fix should be in mailaprop or just in
filtering the input, but anyway we need a reproduction case.

test-data/backslash-problem.mbox

+From nobody Mon Apr 13 10:40:22 2020
+From: J. Random Sender <jrsender@sender.example.com>
+Date: Sat, 11 Apr 2020 15:11:56 -0500
+Message-ID: <V3redN36MUfOrZzPQysYW-QtDcXzQ@example.com>
+Subject: Re: Breakfast Tomorrow!
+To: Some Third Recipient <recipient_three@receivers.example.com>
+Cc: "\"\\\"Lastname1, Firstname1\\\"\"" <first_recipient@receivers.example.com>,
+    "\"\\\"Lastname2, Firstname2\\\"\"" <second_recipient@receivers.example.com>
+
+The original is ~kfogel/mail/breakfast-cabal/chicago/4340.gz FWIW.