The test fails when Tarantool with a renewed schema version recovers from
a gh_8937_data/ snapshot, because `luatest/server_instance.lua` executes:
box.schema.user.grant('guest', 'super', nil, nil, {if_not_exists = true})
while insert into system spaces is forbidden until schema is upgraded.
Fix it by adding `box.schema.user.grant()` to the snapshot.

NO_DOC=test
NO_CHANGELOG=test