Address CH flaky ALTER TABLE queries in tests

What does this MR do and why?

This MR attempts to address flaky CH tests such as this one: https://gitlab.com/gitlab-org/gitlab/-/jobs/6568945062

Assumption: ALTER TABLE DELETE/UPDATE statements are executed async by default. Due to this behavior, tests deleting/updating data might become flaky. By adding the mutations_async setting to the test env we force CH to execute these mutations synchronously.

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