CI: fix to Online DDL flow test, do not t.Log from within a goroutine

[shlomi-noach]

Description

Quick fix to Online DDL flow test flakiness, caused to to logging in a goroutine. We can add more synchronization to ensure the logging is only ever done while the tests is still running, but then again this logging is for convenience only. Switched to fmt.Println instead, it's just as informative.

Related Issue(s)

https://github.com/vitessio/vitess/actions/runs/12676221410/job/35328963706?pr=17443

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[vitess-bot]

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