gspawn, win32: qouted args - escape end backslash
This fixes incorrect behavior on win32 with passing arguments containing spaces and ended in backslash.
The initial commits are adding tests (that initially fails on win32), Later commits fixes problems (and test become passing again).
During resolving first discussions more bugs in existing spawn code for win32 was found and fixed:
- (long present) bug when redirecting only stderr, not stdout
- (long present) subtle problem appearing only while running with coverage - warning from coverage in exiting gspawn-win32-helper.exe was treated as child error output (but it was NOT from the launched process, it was from the helper, so the test fail was "true" fail; actually this issue takes most most delay before coming out with new commits)
- bug in new code, that incorrectly escaped \ before quotes (fix squashed in latest commit)