Request: re-introduce MySQL 5.7 testing for schema.Engine

[shlomi-noach]

Vitess mostly dropped support for MySQL 5.7 and as such we've removed almost all CI tests running on 5.7. However, users will still want to import data from a MySQL 5.7 setup into Vitess. As such, we want to re-introduce some (limited) 5.7 testing.

We propose the tests in schema.Engine as this reads tabel data + sizes etc., required for vreplication.