gst/videotestsrc/gstvideotestsrc.c: move fixation to a fixate function remove negotiate function, basesrc's is good e...

Original commit message from CVS:
* gst/videotestsrc/gstvideotestsrc.c:
(gst_videotestsrc_class_init), (gst_videotestsrc_init),
(gst_videotestsrc_src_fixate):
move fixation to a fixate function
remove negotiate function, basesrc's is good enough
fixes a bug for check when using the element alone