Add a "stop time" feature to chutney

Currently, chutney's test-network.sh waits for the following amount of time:

a mandatory 15 seconds for the consensus to form
up to 60 seconds while verifying the network And then it terminates the network.

But sometimes I want chutney to hang around for several minutes, or even forever.

There should be an option / environmental variable for this "stop time" in test-network.sh.

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