test: Add a test to demonstrate message-sorting regression.

test: Add a test to demonstrate message-sorting regression.

In the recent change to rename threads based on changing subject
lines, I broke message ordering within "notmuch show" output. But our
test suite didn't catch that regressions, because we didn't have any
tests of "notmuch show".

This adds one "notmuch show" test along with the thread-naming
tests. It's not a whole suite of "notmuch show" testing, but it does
catch this regression at least.