test: Add test showing notmuch corrupts a part with a CRLF pair in it

Since commit 2f8871df6ea3c0b44f85a0fc1b4f58a6b70b0a0e notmuch has been
using a function (show_part_content) originally written only for text
parts to save all MIME parts. The problem with this is that this
function converts CRLF pairs to LF only and optionally converts to
UTF-8 encoding. These two conversions have the potential to corrupt
binary data when passed through the function.

This test demonstrates that corruption, and so fails currently, until
we fix the bug.