unittests/clif: add attribute test cases
unittests/clif: add attribute test cases

- missing value in attribute with separator should fail
- single-quoted value in attribute should fail
- empty value in quoted attribute should fail

kaspar030/RIOT,authmillenon/RIOT,ant9000/RIOT,miri64/RIOT,ant9000/RIOT,ant9000/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,miri64/RIOT,OlegHahm/RIOT,kaspar030/RIOT,kaspar030/RIOT,ant9000/RIOT,jasonatran/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,ant9000/RIOT,jasonatran/RIOT,miri64/RIOT,miri64/RIOT,kYc0o/RIOT,OlegHahm/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,OTAkeys/RIOT,authmillenon/RIOT,jasonatran/RIOT,OlegHahm/RIOT,kYc0o/RIOT,jasonatran/RIOT,miri64/RIOT,authmillenon/RIOT,authmillenon/RIOT,OTAkeys/RIOT,jasonatran/RIOT,OTAkeys/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,kYc0o/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,kaspar030/RIOT,authmillenon/RIOT,OTAkeys/RIOT