test/crypto: remove headers more robustly

test/crypto: remove headers more robustly

In [1], Vladimir Panteleev observed that the In-Reply-To and
References headers could be wrapped in the 'default' output format of
notmuch-reply, depending on the version of Emacs creating the
message. In my own experiments notmuch-reply sometimes wraps headers
with only one message-id if that message-id is long enough. However it
happens, this causes the previous approach using grep to fail.

Since I found the proposed unwrapping shell fragment in [1] a bit hard
to follow, I decided to write a little python script instead. Then
Tomi suggested a slight generalization of my script, and here we are.

[1] id:20170817175145.3204-7-notmuch@thecybershadow.net