Adding data migration regression tests for the file backend.
EBookBackendSqliteDB: Fixed upgrade_contacts() for cases with missing vcards.

This patch avoids critical warnings to be fired when opening an old summary
mode SQLite whilst upgrading/migrating an addressbook from EDS <= 3.6.
EBookBackendSqliteDB: Various bug fixes related to upgrades

  o Call introspect_summary() earlier, before ALTER TABLE and
    adding columns of auxilliary data based on fields which
    are in the summary.

  o Fixed ALTER TABLE ADD COLUMN value_translit to actually
    run the statement (seems I had that in place but just missed
    executing the statement).

  o Fetch the system locale if no locale is previously set in
    an existing DB (i.e. upgraded), fixes a case where we access
    a missing ECollator