Skip test galera.MW-402 if debug sync is not available

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• [x] The Jira issue number for this PR is: MDEV-______

Description

Minor fix for MTR test galera.MW-402. Skip it if debug_sync is not compiled into the server.

Release Notes

Nothing.

How can this PR be tested?

Run the test itself

Basing the PR against the correct MariaDB version

• [ ] This is a new feature or a refactoring, and the PR is based against the main branch.
• [x] 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

• [x] 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.