gvdb test: Use g_string_append()
gvdb test: Use g_string_append()

Using g_string_append_c() causes GLib headers to appear in the test
coverage results (since it's an inline function).  Avoid that.

It's probably possible to do this in a more clever way but this works
for now.

GNOME/dconf,GNOME/dconf,GNOME/dconf,GNOME/dconf