itest: add deprecated bbolt db coverage

Expand the kvdb -> SQL migration itest to cover the full post-migration
startup behavior, not just the initial data copy and SQL assertions.

Verify that bbolt startup is blocked once kvdb files are deprecated,
that deleting the SQL database reruns the migration successfully, and
that older litd binaries still fail to start against deprecated kvdb
files.

Also add ordered blocker checks for the deprecated kvdb files so the
test shows startup fails first on accounts.db, then session.db, and
finally rules.db as earlier files are removed.

Author: ViktorT-11