test-sysroot: Use GSystem to spawn subprocess

test-sysroot: Use GSystem to spawn subprocess

I was getting a weird hang in the installed tests with the shell as a
zombie process, not reaped by the parent, which was just stuck in
select() on the output pipes.  The thing is we don't actually want to
capture stdout/stderr, we just want to inherit.

GSystem.Subprocess makes that possible, so let's just use it now that
it's a proper installed library.