Support Cancelleation in PSharpTester

[akashlal]

We should support clean cancellation of PSharpTester. When the user gives a Ctrl-C signal to PSharpTester, it should abort cleanly, reporting whatever it has gathered so far. This involves two changes: (1) stopping the loop that runs test iterations (controlled by the /i flag) and (2) forcing the check that runs a single iteration to its length (controlled by /max-steps flag).to call Stop.

Perhaps we give it a couple of seconds before actually terminating the process. (To make sure that Ctrl-C is respected.)