OC-11191: Increase stability of windows memory leak test measurements

The test was measuring working set of the process before and after using resources that invoked ffi and looking for a variance that might indicate a leak. Changes in ffi from 1.3.1 to 1.5.0 seem to have affected working set (possibly through garbage collection behavior) such that this leak is sometimes triggered.

Working set in general is a less reliable way to detect a leak (it can miss leaks as well as generate false positives), so a switch was made to use the process's total virtual memory allocation as the metric to check.