ges-uri-clip: Add the possibility to specify an assets directory

through the GES_TESTING_ASSETS_DIRECTORY environment variable.
integration: add SeekInfo and get_position callback for seek tests

A Seekinfo structure consists of 2 fields:
- position: the position to seek to
- seeking_position: the position to perform the seek from

Seeks can be appended to a global list e.g. from code:
seeks = g_list_append (seeks, new_seek_info (0.2 * GST_SECOND, 0.6 * GST_SECOND));
seeks = g_list_append (seeks, new_seek_info (1.0 * GST_SECOND, 1.2 * GST_SECOND));
seeks = g_list_append (seeks, new_seek_info (1.5 * GST_SECOND, 1.8 * GST_SECOND));

The get_position callback checks the current position and attempts to perform
the corresponding seek with gst_element_seek_simple