internal/fuzz: fix race between context deadline and error suppression

[klauspost]

When the coordinator's context deadline expires, there is a brief
window where ctx.Err() is set but fuzzCtx (a child context) has not
yet been canceled. If the coordinator's event loop observes <-doneC
during this window, the stop function compares the error against
fuzzCtx.Err() which is still nil, so the deadline error is not
suppressed and leaks out as a spurious test failure.

Fix this by also comparing against ctx.Err() in the suppression check.
ctx.Err() is guaranteed to be set before ctx.Done() is closed because
cancelCtx.cancel stores the error before closing the done channel.

Fixes #75804

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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[gopherbot]

Message from Klaus Post:

Patch Set 2:

(1 comment)

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