systemclock: Clean up the tests a bit.
systemclock: Clean up the tests a bit.

Add some cleanups to the system clock tests, to free all the memory and
unschedule/unref all clock IDs we allocate.

Use a mutex in one test to avoid potential threading problems on multicore
machines.

lubosz/gstreamer,krichter722/gstreamer,krieger-od/gstreamer,centricular/gstreamer,drothlis/gstreamer,drothlis/gstreamer,justinjoy/gstreamer,Distrotech/gstreamer,lovebug356/gstreamer,krichter722/gstreamer,ylatuya/gstreamer,mparis/gstreamer,mparis/gstreamer,lovebug356/gstreamer,krichter722/gstreamer,mrchapp/gstreamer,StreamUtils/gstreamer,mrchapp/gstreamer,ahmedammar/platform_external_gst_gstreamer,lubosz/gstreamer,ylatuya/gstreamer,StreamUtils/gstreamer,lubosz/gstreamer,collects/gstreamer,jpakkane/gstreamer,mparis/gstreamer,ahmedammar/platform_external_gst_gstreamer,ahmedammar/platform_external_gst_gstreamer,krieger-od/gstreamer,lubosz/gstreamer,krieger-od/gstreamer,shelsonjava/gstreamer,mparis/gstreamer,centricular/gstreamer,centricular/gstreamer,ylatuya/gstreamer,Distrotech/gstreamer,drothlis/gstreamer,cfoch/gstreamer,collects/gstreamer,collects/gstreamer,magcius/gstreamer,jpakkane/gstreamer,ylatuya/gstreamer,ylatuya/gstreamer,surround-io/gstreamer,krichter722/gstreamer,Lachann/gstreamer,jpxiong/gstreamer,ensonic/gstreamer,jpxiong/gstreamer,cfoch/gstreamer,shelsonjava/gstreamer,krieger-od/gstreamer,Lachann/gstreamer,shelsonjava/gstreamer,lovebug356/gstreamer,jpakkane/gstreamer,cablelabs/gstreamer,magcius/gstreamer,justinjoy/gstreamer,magcius/gstreamer,cablelabs/gstreamer,mrchapp/gstreamer,Lachann/gstreamer,ahmedammar/platform_external_gst_gstreamer,cfoch/gstreamer,krieger-od/gstreamer,cablelabs/gstreamer,StreamUtils/gstreamer,surround-io/gstreamer,jpxiong/gstreamer,lovebug356/gstreamer,Lachann/gstreamer,mrchapp/gstreamer,ensonic/gstreamer,collects/gstreamer,lubosz/gstreamer,magcius/gstreamer,ahmedammar/platform_external_gst_gstreamer,lovebug356/gstreamer,ensonic/gstreamer,justinjoy/gstreamer,Distrotech/gstreamer,drothlis/gstreamer,mrchapp/gstreamer,Lachann/gstreamer,StreamUtils/gstreamer,ensonic/gstreamer,collects/gstreamer,justinjoy/gstreamer,drothlis/gstreamer,mparis/gstreamer,jpxiong/gstreamer,StreamUtils/gstreamer,shelsonjava/gstreamer,krichter722/gstreamer,jpxiong/gstreamer,surround-io/gstreamer,magcius/gstreamer,Distrotech/gstreamer,cablelabs/gstreamer,cfoch/gstreamer,cfoch/gstreamer,surround-io/gstreamer,ensonic/gstreamer,jpakkane/gstreamer,Distrotech/gstreamer,shelsonjava/gstreamer,centricular/gstreamer,cablelabs/gstreamer,jpakkane/gstreamer,centricular/gstreamer,justinjoy/gstreamer,surround-io/gstreamer