tests: fix locking in gstpoll unit test
tests: fix locking in gstpoll unit test

The mutex needs to be locked when g_cond_wait*() is
called.

Lachann/gstreamer,surround-io/gstreamer,Lachann/gstreamer,jpxiong/gstreamer,jpxiong/gstreamer,centricular/gstreamer,centricular/gstreamer,StreamUtils/gstreamer,shelsonjava/gstreamer,StreamUtils/gstreamer,shelsonjava/gstreamer,jpxiong/gstreamer,jpakkane/gstreamer,Lachann/gstreamer,surround-io/gstreamer,mparis/gstreamer,surround-io/gstreamer,mparis/gstreamer,jpakkane/gstreamer,mparis/gstreamer,jpakkane/gstreamer,StreamUtils/gstreamer,surround-io/gstreamer,jpakkane/gstreamer,centricular/gstreamer,centricular/gstreamer,shelsonjava/gstreamer,jpxiong/gstreamer,jpakkane/gstreamer,Lachann/gstreamer,krichter722/gstreamer,krichter722/gstreamer,StreamUtils/gstreamer,mparis/gstreamer,Lachann/gstreamer,mparis/gstreamer,jpxiong/gstreamer,krichter722/gstreamer,centricular/gstreamer,shelsonjava/gstreamer,shelsonjava/gstreamer,krichter722/gstreamer,StreamUtils/gstreamer,surround-io/gstreamer,krichter722/gstreamer