Extend gst-harness so it is possible to get buffer lists provided by an harnessed element and verify if an harnessed element uses BufferLists or separate Buffers when pushing data.
This patch adds the gst_harness_push_list, gst_harness_pull_list and the gst_harness_try_pull_list functions, in addition to setting the gst_pad_set_chain_list_function on the harnessed GstElement. It also ensures that existing tests that does not use these functions still works as before, even with elements that supports buffer lists.