Make sure our checks and the videotestsrc plugin link against the local uninstalled...

Make sure our checks and the videotestsrc plugin link against the local uninstalled gst libs and not any installed gs...

Original commit message from CVS:
* gst/videotestsrc/Makefile.am:
* tests/check/Makefile.am:
Make sure our checks and the videotestsrc plugin link against the
local uninstalled gst libs and not any installed gst libs that
might happen to exist as well.
* tests/check/elements/adder.c: (message_received),
(test_event_message_received), (test_play_twice_message_received):
* tests/check/elements/ffmpegcolorspace.c: (GST_START_TEST):
Fix compiler warnings when compiling against core with disabled
debugging system.