tests: Add missing unrefs of objects after use

Unreffing the objects returned by gst_bin_get_by_name() and
gst_pipeline_get_use() were missing in several tests, so add these.

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=734359
rtpbasepayload: Implement video SDP attributes

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=726472