The main goal is to support gzipped output for future internal
calls (e.g. from notmuch-new) to notmuch_database_dump.
The additional dependency is not very heavy since xapian already pulls
in zlib.
We want the dump to be "atomic", in the sense that after running the
dump file is either present and complete, or not present. This avoids
certain classes of mishaps involving overwriting a good backup with a
bad or partial one.
All test scripts to be executed are now named as T\d\d\d-name.sh,
numers in increments of 10.
This eases adding new tests and developers to see which are test scripts
that are executed by test suite and in which order.
