test: add database upgrade test from format version 1 to 2

Test the upgrade from probabilistic to boolean folder: terms, and
addition of path: terms.

The test depends on the pre-built test corpus and database tarball and
checksum file being in place. If it's not, the test is skipped. The
mechanism to fetch the test database will be added later.

At the time of writing, a working test database and checksum file is
available at

   http://notmuchmail.org/releases/test-databases/

It has been noted that some non-GNU environments make lack
sha256sum. We leave this portability issue for a followup patch.