gtestutils: make the new assert messages more detailed
gtestutils: make the new assert messages more detailed

g_assert_true(), g_assert_false(), g_assert_null(), and
g_assert_nonnull() simply printed out the expression they were
checking, without any further explanation of what went wrong. (In
particular, "g_assert_true(x)" and "g_assert_false(x)" would both
print the same thing on failure.) Add a little bit more context.

https://bugzilla.gnome.org/show_bug.cgi?id=724385

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