gst/videotestsrc/: Make videotestsrc a pushsrc.

gst/videotestsrc/: Make videotestsrc a pushsrc.

Original commit message from CVS:
* gst/videotestsrc/Makefile.am:
* gst/videotestsrc/gstvideotestsrc.c: (gst_videotestsrc_get_type),
(gst_videotestsrc_class_init), (gst_videotestsrc_negotiate),
(gst_videotestsrc_setcaps), (gst_videotestsrc_getcaps),
(gst_videotestsrc_init), (gst_videotestsrc_event),
(gst_videotestsrc_create), (gst_videotestsrc_start),
(gst_videotestsrc_stop), (gst_videotestsrc_get_times),
(gst_videotestsrc_set_pattern), (gst_videotestsrc_set_property),
(gst_videotestsrc_get_property):
* gst/videotestsrc/gstvideotestsrc.h:
Make videotestsrc a pushsrc.