`checker`: Remove `{}` in `test_err()`

See merge request !2

The `test_err()` function was using a lingering artifact (`{}`) from the
use of `xargs` in other tests. This failed the test.

This commit removes the lingering artifact. The test now passes.

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

Enable Docker for all stages correctly

See merge request !1