tests: collectpads: add flushing seek tests
tests: collectpads: add flushing seek tests

https://bugzilla.gnome.org/show_bug.cgi?id=708416

Lachann/gstreamer,krichter722/gstreamer,shelsonjava/gstreamer,lovebug356/gstreamer,Lachann/gstreamer,mparis/gstreamer,jpakkane/gstreamer,StreamUtils/gstreamer,jpxiong/gstreamer,jpxiong/gstreamer,StreamUtils/gstreamer,shelsonjava/gstreamer,shelsonjava/gstreamer,cfoch/gstreamer,StreamUtils/gstreamer,lovebug356/gstreamer,lubosz/gstreamer,ensonic/gstreamer,Lachann/gstreamer,surround-io/gstreamer,surround-io/gstreamer,lovebug356/gstreamer,lubosz/gstreamer,lovebug356/gstreamer,cablelabs/gstreamer,lubosz/gstreamer,krichter722/gstreamer,jpxiong/gstreamer,cablelabs/gstreamer,shelsonjava/gstreamer,ensonic/gstreamer,mparis/gstreamer,Lachann/gstreamer,cfoch/gstreamer,krichter722/gstreamer,ensonic/gstreamer,lubosz/gstreamer,cfoch/gstreamer,jpakkane/gstreamer,StreamUtils/gstreamer,cfoch/gstreamer,mparis/gstreamer,jpakkane/gstreamer,surround-io/gstreamer,ensonic/gstreamer,cablelabs/gstreamer,centricular/gstreamer,krichter722/gstreamer,centricular/gstreamer,mparis/gstreamer,jpakkane/gstreamer,ensonic/gstreamer,surround-io/gstreamer,lubosz/gstreamer,shelsonjava/gstreamer,jpxiong/gstreamer,lovebug356/gstreamer,cfoch/gstreamer,krichter722/gstreamer,centricular/gstreamer,jpakkane/gstreamer,surround-io/gstreamer,jpxiong/gstreamer,Lachann/gstreamer,cablelabs/gstreamer,cablelabs/gstreamer,StreamUtils/gstreamer,centricular/gstreamer,centricular/gstreamer,mparis/gstreamer