test(nns): Increase timeout for rent_subnet_test

[jasonz-dfinity]

The rent_subnet_test often times out. From those tests that did time out, it seems they mostly finished testing (and passes, e.g. this one) while they might not have enough time to clean up. Therefore, the increase of timeout isn't so aggressive - we are doubling the overall timeout from 15m->30m

[github-actions]

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