gtestutils: make g_test_suite_run{,internal} less confusing
gtestutils: make g_test_suite_run{,internal} less confusing

Rewrite g_test_suite_run() and g_test_suite_run_internal() to make it
clearer what they do (while still preserving exact backward
compatibility, meaning we need to handle the "-p" case differently
from the non-"-p" case).

https://bugzilla.gnome.org/show_bug.cgi?id=754284

endlessm/glib,endlessm/glib,MathieuDuponchelle/glib,mzabaluev/glib,cention-sany/glib,endlessm/glib,johne53/MB3Glib,ieei/glib,endlessm/glib,Distrotech/glib,cention-sany/glib,ieei/glib,Distrotech/glib,johne53/MB3Glib,Distrotech/glib,johne53/MB3Glib,johne53/MB3Glib,MathieuDuponchelle/glib,MathieuDuponchelle/glib,johne53/MB3Glib,MathieuDuponchelle/glib,tchakabam/glib,MathieuDuponchelle/glib,cention-sany/glib,endlessm/glib,mzabaluev/glib,mzabaluev/glib,tchakabam/glib,Distrotech/glib,tchakabam/glib,cention-sany/glib,ieei/glib,tchakabam/glib,ieei/glib,cention-sany/glib,ieei/glib,johne53/MB3Glib,mzabaluev/glib,mzabaluev/glib,tchakabam/glib,Distrotech/glib