filesink: Use SYNC_AFTER flag in seeking test
filesink: Use SYNC_AFTER flag in seeking test

Otherwise it's not guaranteed that buffers are actually on disk after
pushing them, and reading the file via g_file_get_contents() might not
include them yet.

StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer