test/unit/merge: Fix up-to-date #migrateFileid() test
test/unit/merge: Fix up-to-date #migrateFileid() test

Make sure doc is actually up-to-date.
Also reuse `BaseMetadataBuilder#noFileid()`.

cozy-labs/cozy-desktop,cozy-labs/cozy-desktop,cozy-labs/cozy-desktop,cozy-labs/cozy-desktop