fix: mark test_dml_on_global_tbls flaky

Summary

• fix: mark test_dml_on_global_tbls flaky test with xfail until we move on new vshard version (which should fix problem with inconsistent reads)

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• Cherry-pick to: none
• Docs follow-up: not necessary

requested review from @d.rodionov

mentioned in issue #1025 (closed)

approved this merge request

merged