This commit adds a new feature to the network harness: test writers are
now able to select arbitrary nodes, causing them to restart.
This functionality will be useful in the future in order to test
scenarios like persisting data across restarts, re-syncing after
re-connections, reacting to the counter party broadcasting revoked
states, etc.
This commit adds some new networkHarness helper methods which are
mean’t to reduce the verbosity of the previous basic tests, and also to
enable developers to right tests mote easily five a higher level
interface.