Make test suite more powerful

Feature Request

Is your feature request related to a problem? Please describe:

In /cmd/explaintest we have a test suite like mysqltest, however, we does not support all of its features, like change users, error checks and many other features.

TODO: list of unsupported features

Describe the feature you'd like:

https://dev.mysql.com/doc/dev/mysql-server/latest/PAGE_MYSQLTEST_LANGUAGE_REFERENCE.html

Maybe we does not need all the features of mysqltest.

Teachability, Documentation, Adoption, Migration Strategy:

Since this is just a test suite, maybe it is not necessary to be compatible with old versions?

Also we need a document for writing such tests.