This makes the test suite bash-specific, but that's not much of
an issue for me, (if somebody else would prefer some other language
then they can rewrite the test suite and maintain it).
The advantage here is that we'll now be able to easily generate
custom messages for testing operations that depend on the message
content, (such as "notmuch reply", etc.).
This notmuch-test script simply runs a few different notmuch operations,
(things that I found were useful while testing the rename-support code).
It's not useful as a test suite yet, since it doesn't actually check
the results of any operation, (the user of the suite has to know what
the results should be and must manually verify them. So there's no
integration with the build system yet, (no "make test" target).
But I didn't want to lose what I had so far, so here it is.
