utils: Unref/release pads in error cases when linking pads

Previously gst_element_link_pads_full() forgot to unreference or release
request pads in several error cases. Also comments were added mentioning
why releasing is not necessary in some places.

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=733119
tests: Add test verifying gst_element_link_pads_full()

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=733119