MDEV-33952: Fix flaky galera_create_table_as_select test with debug sync

[denis-protivensky]

• The Jira issue number for this PR is: MDEV-33952

Description

The test that triggers multi-master conflict between two CTAS commands uses LOCK/UNLOCK TABLES to block local CTAS from progress. It could result in a race when UNLOCK TABLES command is issued a bit earlier then needed, causing local CTAS to run further and change wsrep transaction state, so that a different code path is taken later and the original error gets overridden, causing the test to fail. The solution is to replace LOCK/UNLOCK TABLES with debug sync points.

Release Notes

The fix is for 10.5+ versions.

How can this PR be tested?

MTR test is fixed an re-enabled.

Basing the PR against the correct MariaDB version

• This is a new feature and the PR is based against the latest MariaDB development branch.
• This is a bug fix and the PR is based against the earliest maintained branch in which the bug can be reproduced.

PR quality check

• I checked the CODING_STANDARDS.md file and my PR conforms to this where appropriate.
• For any trivial modifications to the PR, I am ok with the reviewer making the changes themselves.

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