The spawn_test is enabled on win32 meson build, both msys and msvc.

Some modifications to make it useful for auto-testing on win32:
- use own argv0 to find helper win32-specific subprogram
- helper subprogram and conditions changed, so testing is fully
automated instead of manually checking contents of some MessageBoxes

Redirection test checks "sort" output for locale-independent string
instead of relying on "netstat" locale-dependent string.
Also with "sort" it become usable on unix, so enabled there too.
Currently this fails on win32 with coverage since
some coverage-realted error output from gpawn-win32-helper
is unexpectedly treated as executed subprocess output.


Added test checking "sort" with error-only redirection. This also fails
on win32 by now, due to a typo in gspawn-win32.c (checks for stdout
redirection instead of stderr)