Add run-time watchdog timeout override

[joshrsmith]

It would be nice if inside a test case (at runtime) the watchdog timer could be overridden to a new value.

I have had a few instances where some test cases take orders of magnitude more simulation time than the rest of the test cases. So you could set the global time based on that one test case, at the expense of the simulation taking much longer to fail if something goes wrong in one of the short test cases.

This may be related to #319.