gsubprocess: Add a missing test for invalid UTF-8 output
gsubprocess: Add a missing test for invalid UTF-8 output

There were tests for invalid UTF-8 output when asynchronously
communicating with a subprocess, but nothing for synchronous
communication. Add such a test, and refine the code as a result.

Signed-off-by: Philip Withnall <29c2a2ac2a854b9cf047275aa9de346d096be2e0@endlessm.com>

endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib