Test we can ALTER log tables directly when not being written
to.

This removes the contraint in the rpl_mysql_upgrade.test such
that we can run mysql_upgrade --write-binlog all the way
through to a replica. We test this in the replication scenario
where the mysql.{slow,general}_log tables aren't being
written to.

Reviewers: Vicențiu Ciorbaru, Anel Husakovic