gsubprocess: Fix error detection logic in tests
gsubprocess: Fix error detection logic in tests

Various tests were depending on local_error being set by a callback
when it could never have been the case. Simplify async error detection
logic in those cases, and fix leak of GError.

https://bugzilla.gnome.org/show_bug.cgi?id=711802

tamaskenez/glib,Distrotech/glib,mzabaluev/glib,lukasz-skalski/glib,mzabaluev/glib,endlessm/glib,cention-sany/glib,MathieuDuponchelle/glib,johne53/MB3Glib,tamaskenez/glib,MathieuDuponchelle/glib,endlessm/glib,johne53/MB3Glib,mzabaluev/glib,tchakabam/glib,krichter722/glib,gale320/glib,tamaskenez/glib,MathieuDuponchelle/glib,gale320/glib,tchakabam/glib,tchakabam/glib,lukasz-skalski/glib,tamaskenez/glib,ieei/glib,tamaskenez/glib,johne53/MB3Glib,endlessm/glib,ieei/glib,johne53/MB3Glib,endlessm/glib,cention-sany/glib,MathieuDuponchelle/glib,ieei/glib,krichter722/glib,johne53/MB3Glib,krichter722/glib,cention-sany/glib,lukasz-skalski/glib,mzabaluev/glib,MathieuDuponchelle/glib,Distrotech/glib,endlessm/glib,johne53/MB3Glib,ieei/glib,cention-sany/glib,mzabaluev/glib,krichter722/glib,lukasz-skalski/glib,cention-sany/glib,tchakabam/glib,gale320/glib,Distrotech/glib,krichter722/glib,Distrotech/glib,gale320/glib,gale320/glib,lukasz-skalski/glib,Distrotech/glib,tchakabam/glib,ieei/glib