gltestsrc: Use default get_caps implementation

gltestsrc: Use default get_caps implementation

The custom code is wrong as it ignores the templates, which leads to
missing fields in the result. Instead, simply use the default get_caps
implementation which does it correctly (get the template, intersect
with filter and return).

https://bugzilla.gnome.org/show_bug.cgi?id=749237