tests: Add test that Samba cannot be started with a backup DB

tests: Add test that Samba cannot be started with a backup DB

We don't want users to take a backup file, and then simply untar it and
run Samba (Several modifications to the DB need to be made as part of
the restore process, so users should always run the 'backup restore'
command).

To enforce this, prime_ldb_databases() now refuses to start Samba if the
backupDate marker is present in the DB. This patch adds a test-case that
proves this basic behaviour works.

Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>