Avoid nctl tests running indefinitely

Example of failed test which was killed by the runner: https://drone-auto-casper-network.casperlabs.io/casper-network/casper-node/8676/1/4. We should ensure that all nctl tests which use this function to wait for a number of eras to pass have a hard timeout, so that the tests fail rather than being killed, as we get no log dumps when a test is killed by the runner.