Fix OpenFileHandleTest's GetHandleCountForCurrentProcess() function on Mac

[achristoforides]

For all the tests in OpenFileHandleTest.cpp, they are using the GetHandleCountForCurrentProcess() function to get the current number of open file handles for the current process. On Windows and Linux, this appears to be working correctly while on Mac it isn't returning the correct number after opening a new file handle in the test even though the system calls for opening and closing a file handle are present in the dtruss trace of the application run.