tests: add 2 unit tests for non-flush seek with gstbaseparse

tests: add 2 unit tests for non-flush seek with gstbaseparse

The unit test defines a test parse element that inherit from GstBaseParse.
The test pipeline is: fakesrc ! testparse ! fakesink sync=1

Before the fix b2c05cac8 the first new test would have fail because the
pipeline would have wait doing nothing just after proceeded the seek event.
The second new test would have fail because the pipeline would have
played the media instantly just after proceeded the seek event
(like if sync was FALSE on the sink).

https://bugzilla.gnome.org/show_bug.cgi?id=777780