fix: set kestrel block time to 1 second

[rflechtner]

The kestrel block time was set to 6s on the level of the aura config, which apparently was overridden by the MinimumPeriod of the timestamp pallets though.
Now the aura config takes precedence as it looks like, and block time (even when using the --dev flag) is 6 seconds.
This means that tests take much longer and sometimes time out.

Reducing the aura config to 1s helps. Not sure we have any other contexts in which this runtime was used, and where we maybe expected a 6s block time?

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Checklist:

• I have verified that the code works
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