tests: only call check-code once

The accepted warnings list is now empty, let's try to keep it that way.

tests/test-check-code-hg.t

@@ -1,10 +1,11 b''
   $ check_code="$TESTDIR"/../contrib/check-code.py
   $ cd "$TESTDIR"/..
   $ if hg identify -q > /dev/null; then :
   > else
   >     echo "skipped: not a Mercurial working dir" >&2
   >     exit 80
   > fi
-  $ hg manifest | xargs "$check_code" || echo 'FAILURE IS NOT AN OPTION!!!'
 
-  $ hg manifest | xargs "$check_code" --warnings --nolineno --per-file=0 || true
+New errors are not allowed. Warnings are strongly discouraged.
+
+  $ hg manifest | xargs "$check_code" --warnings --nolineno --per-file=0