Add --keep option to keep any remaining stuff in index or file. We
could distinguish between failures to index and failures to apply tags
or maildir sync, but for simplicity just have 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.