onlineddl_vrepl_stress: fix flakiness caused by timeouts

[shlomi-noach]

Description

This PR fixes once observed flakiness in onlineddl_vrepl_stress CI test, caused by a timeout. The main fix here is to exit when the context is cancelled, before checking the error code.
While at it, I've simplified the timeout mechanism to rely solely on context.WithTimeout() or context.WithCancel() and removed some superfluous code.

Related Issue(s)

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[rohit-nayak-ps]

Since we are handling ctx.Done() just above and returning, when will it come here with a DeadlineExceeded?

[shlomi-noach]

You're absolutely right. Removing the if condition.

[shlomi-noach]

done