test: fix #6310 upgrade test

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

test/box-luatest/gh_6310_grant_rw_access_on__session_settings_space_to_public_role_test.lua

@@ -49,7 +49,6 @@ g.test_upgrade = function()
     g.upgrade:exec(function()
         local t = require('luatest')
 
-        box.schema.upgrade()
         local _session_settings_privs
         local public_privs = box.schema.role.info('public')
         for _, priv in pairs(public_privs) do