Add a very rough beginning of a test suite.

Add a very rough beginning of a test suite.

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.