runtests.pl: make sure exit value takes account of non-standard test failures.
MultiTestDB: allow for driver-specific, table-specific SQL to be run before and after load_txt_dump().

From files <dump_dir>/<driver>/<table>.{pre,post}.
Oops, don't forget to mirror necessary changes into mysql driver!

MultiTestDB: allow for driver-specific, table-specific SQL to be run before and after load_txt_dump().
Merge pull request #3 from Anacode/master

runtests.pl: ensure exit code is not over-optimistic; MultiTestDB: SQLite munging