checker: Add checker, then linter

First run `make check`, then `make lint`. This is useful because Moodle
only extract the first 300 lines of output. If `make lint` is first,
that output is typically covered by the linter output, leaving out
students from seeing the checker output (which is more important).

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