The upgrade test worked incorrectly: the instances WAL directory contained
xlog with the latest schema version and the snapshot contained an older
schema version (2.7.1) than required (2.9.1), whereas there only need to
be a snapshot with 2.9.1 schema version — fix this descrepancy.

Also, schema upgrade is performed automatically: remove the redundant
`box.schema.upgrade` call.

NO_CHANGELOG=test fix
NO_DOC=test fix
NO_DOC=test fix