Create a new console for each test worker on Windows

Create a new console for each test worker on Windows

The primary motivation to have each test worker running its own console
is to allow the windows_mb_path tests to run in parallel.  A nice side
effect is that this also prevents changing the code page of the
tester's console window (which can even cause its font to be changed).

To be able to do so, we introduce the `create_new_console` option for
`proc_open()`, which might occasionally be useful for other purposes
than testing.