tests: fdsrc: don't ignore return value of write()
tests: fdsrc: don't ignore return value of write()

Causes compiler warnings on some systems.

shelsonjava/gstreamer,StreamUtils/gstreamer,centricular/gstreamer,StreamUtils/gstreamer,centricular/gstreamer,mparis/gstreamer,centricular/gstreamer,jpxiong/gstreamer,mparis/gstreamer,krichter722/gstreamer,mparis/gstreamer,shelsonjava/gstreamer,mparis/gstreamer,jpxiong/gstreamer,krichter722/gstreamer,jpxiong/gstreamer,shelsonjava/gstreamer,jpxiong/gstreamer,StreamUtils/gstreamer,centricular/gstreamer,krichter722/gstreamer,mparis/gstreamer,krichter722/gstreamer,centricular/gstreamer,krichter722/gstreamer,shelsonjava/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,shelsonjava/gstreamer,jpxiong/gstreamer