Use an integer when specifying the XWorkItem wait rather than a TimeSpan to avoid a Windows casting issue in SmartThreadPool for large TimeSpans.

TimeSpan.Milliseconds is an int64.  However, STP casts this to an int (32-bit).
If TimeSpan.MaxValue is given then the casting results in an invalid value for the SDK WaitHandle.WaitAll() call.
This was causing the co-op script termination regression tests to fail on Windows but not Mono 2.10.8 (which is perhaps not strict in the negative values that it accepts).
Solution here is to use the int millisecondsTimeout STP call rather than the TimeSpan one.
This also allows us to more clearly specify Timeout.Infinite rather than TimeSpan.MaxValue
Thanks to Teravus for this spot.