logs are not being flushed to run.out

[nonsense]

It appears that logs are not flushed when we write to stdout - if we run a test plan on cluster:k8s we see output in stdout for given test instances via kubectl logs, but those logs do not appear immediately in run.out on the S3 bucket.

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There are two parts to this:

1. flushing on s3bucket daemon (which runs s3fs)
2. flushing within the testground codebase

[raulk]

We're likely missing a call to https://godoc.org/go.uber.org/zap#Logger.Sync when the instance shuts down. This should be solved by calling re.logger.SLogger().Sync() from inside RunEnv#Close().

[nonsense]

This is not just about flushing at the end of the test. We should flush periodically or after every call to the logger, because if a test deadlocks, we want to be able to get the last emitted logs, even if the buffer is not full.