Add MTR tests for MySQL (5.7) to MariaDB migration

Currently upgrades from MySQL 5.7 to MariaDB fail on certain edge
cases. This commit adds MTR tests to reproduce those issues. The purpose
of this is to make the problems visible and facilitate the development
of the proper upgrade code in MariaDB. The MTR tests are added to skip
list as they will fail for all branches until these incompatibilities
are addressed.

Populated a MySQL 5.7 data directory from official MySQL docker image.
The scripts are stored in mysql-test/lib, and the data directory is
stored in std_data to be consumed by MTR directly.

Add MTR tests for the usual migration procedure from MySQL to MariaDB.

Start MariaDB from the MySQL data directory
Run mariadb-upgrade
Verify that the upgrade succeed
Also highlight the incompatibilities that are either not documented
before or documented but the detailed scenario are not clarified. These
tests will fail for now but can be used as references for future to
improve these incompatibilities.

Focus on MySQL 5.7 migration for now.

Pull request: https://github.com/MariaDB/server/pull/2170