state: add regression test for Node slice persistence

Drives the persist path for ApprovedRoutes, Tags and Endpoints —
seed a non-empty value, clear to nil, read the column back from disk,
then close the State and reopen one against the same sqlite file to
simulate a server restart. Pins the contract the named IsZero slice
types enforce so future changes to the persist path cannot silently
drop a cleared slice column.

Updates #3110