harness: Improve detection of element type

harness: Improve detection of element type

The element flag does not indicate wether a bin should be tested as a
source or as a sink, eg. a bin with the sink flag may still have a
source pad and a bin with the source flag may have a sink pad. In this
case it is better to determine the element type by looking at the
available pads and pad templates.

Also rename srcpad and sinkpad where it actually represents
element_srcpad_name and element_sinkpad_name.

https://bugzilla.gnome.org/show_bug.cgi?id=752493