tests: Fix use of linter.out when grading

Linter tools (`cpplint`, `checkpatch`, `shellcheck`) should use
`linter.out` as their output file. Fix the `grade.sh` script to use
`linter.out`, instead of the old, wrong `checker.out`.

Signed-off-by: Razvan Deaconescu <razvan.deaconescu@upb.ro>