gtestutils: better diagnostics if a captured subprocess fails
gtestutils: better diagnostics if a captured subprocess fails

It's unhelpful to get an error saying that stderr didn't match a
desired pattern, or matched an undesired pattern, without also
telling you what *was* on stderr. Similarly, if a test subprocess
exits 1, there's probably something useful on its stderr that
could have told you why.

Bug: https://bugzilla.gnome.org/show_bug.cgi?id=748534
Signed-off-by: Simon McVittie <2b470b34502dbf3ba4bcc04ea7e64f4e0e104f33@collabora.co.uk>
Reviewed-by: Dan Winship <a4113c7b8a2a3e6b96e702b23e93a123ab762619@gnome.org>

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