tests: add more tests for datetime value serialisation/deserialisation
tests: add more tests for datetime value serialisation/deserialisation

Esp. of partial datetimes.

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