gtestdbus: Fix variable shadowing

Shut up, GCC.

https://bugzilla.gnome.org/show_bug.cgi?id=712148
gtestdbus: Fix non-const use of const variables

The argv array should be declared as const.

https://bugzilla.gnome.org/show_bug.cgi?id=712148
gtestdbus: Minor documentation fixes

https://bugzilla.gnome.org/show_bug.cgi?id=712148