MDEV-16809 Allow full redo logging for ALTER TABLE

MDEV-16791 mariabackup : allow consistent backup, in presence of concurrent DDL,

                      also without --lock-ddl-per-table

point to the need to have sufficient corresponding test.

MTR based tests could be only used for some basic coverage.

Whereas exemplary tests are easy written and recommended to have this way cannot be

used as main method for providing sufficient functional coverage.

a) Developing sufficient MTR tests for mariabackup with concurrent DDL is quite costly because the amount of relevant DDL is huge.

b) Even if a) would be cheap than running these tests frequent like as members of the

    regression tests would require too much elapsed runtime and resources (assume CPU)

Given the fact that we already have some crowd of RQG tests checking roughly all kinds of concurrent DDL it seems to be recommended to do the following with RQG:

- the server is under serious load by arbitrary DDL (existing YY grammars)

- run parallel in a loop

  1. Hot backup of data of running server 1

  2. Prepare the data backed up for restore + restore to other place than server 1

  3. Start some additional server 2 on that data

  4. Run check table in server 2

  5. If no trouble during 4. shutdown of server 2 + destroy his data + run next loop round.

      If trouble terminate the test. 

It is not 100% clear if the loop should be implemented as

- periodic reporter (management of the server 2 might be complicated)

- feature of the RQG runner