Develop a successor for the RQG grammar alter_online, Improve RQG

We lack some some sufficient thorough test which focuses on concurrent execution
of any (1) table related DDL and DML only (2).
(1) Even a DDL like DROP SCHEMA or TABLESPACE could affect tables.
(2) Optimizer and stored program language related bugs are not in scope in order
to make the test efficient during execution, failure analysis and replay.
Features already existing or being in development like
https://jira.mariadb.org/projects/MDEV/issues/MDEV-13134
will get by that more functional coverage.
The existing RQG grammar conf/runtime/alter_online.* would be some promising base
for such a test.