Modify the "result" target to actually display proof results.

"make result" is very useful for checking individual proofs.
This commit amends this Makefile target to display the last
4 lines of logs/results.txt following by any lines
that contain the string "FAILURE".

Signed-off-by: Rod Chapman <rodchap@amazon.com>

Author: rod-chapman

proofs/cbmc/Makefile.common

@@ -953,6 +953,9 @@ result:
 	$(MAKE) -B _result
 	@ echo Running 'litani build'
 	$(LITANI) run-build
+	tail -4 $(LOGDIR)/result.txt
+	-grep FAILURE $(LOGDIR)/result.txt
+
 
 _property: $(LOGDIR)/property.xml
 property: