stop test cluster with INT rather than KILL

since writing tests is supposed to ultimately alleviate programmer anxiety, rather than increase it, this PR sends the test cluster process a INT (please stop) signal rather than the harsher KILL (you are dead) signal. 
The KILL signal will cause the parent shell to exit with a scary "killed process N ...." with full command thereafter, causing lots of verbage in the terminal on what should be a normal, non-failure, process completion.

I'm not entirely sure of the history of using KILL (other than probably avoiding hung processes), but in my tests on various *nix flavors, INT works just fine. The docs claim it works on Windows too, though I have not tried.