Fix helper for determining system memory usage on Windows

On Windows systems in other languages than English, the method 
'Magento_Test_Helper_Memory::getWinProcessMemoryUsage()' calls 
'Magento_Test_Helper_Memory::convertToBytes()' with an invalid 
argument, because the column names from the output of 'tasklist' 
are language specific.

The alternative usage of the column index seems to be stable over 
the various versions of Windows, where 'tasklist' is available. 
Tested on Windows 8

related #237