In reverse playback we were not taking into account the current buffer
samples to check if we had reached EOS which was leading to a buffer
with PTS = CLOCK_TIME_NONE containing too many frames followed by a
useless buffer with pts=0 duration=0, and a g_critical issue in
gst_object_sync_values.
Also add a validate based test case.
Without that patch this is how the expectation fails:
``` diff
--- log-asink-sink-expected 2020-05-22 23:22:42.654384579 -0400
+++ log-asink-sink-actual 2020-05-22 23:29:35.671586380 -0400
@@ -27,5 +27,6 @@
buffer: pts=0:00:00.058820861, due=0:00:00.023219955, flags=discont
buffer: pts=0:00:00.035600907, due=0:00:00.023219954, flags=discont
buffer: pts=0:00:00.012380952, due=0:00:00.023219955, flags=discont
-buffer: pts=0:00:00.000000000, due=0:00:00.012380952, flags=discont
+buffer: due=0:00:00.012380953, flags=discont
+buffer: pts=0:00:00.000000000, flags=discont
event eos: (no structure)
```
Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/-/merge_requests/667>
