tests/shell: record the test duration (wall time) in the result data

tests/shell: record the test duration (wall time) in the result data

Runtimes are important. Add a way to find out how long tests took.

  $ ./tests/shell/run-tests.sh
  ...
  $ for d in /tmp/nft-test.latest.*/test-*/ ; do \
         printf '%10.2f  %s\n' \
                "$(sed '1!d' "$d/times")" \
                "$(cat "$d/name")" ; \
    done \
    | sort -n \
    | awk '{print $0; s+=$1} END{printf("%10.2f\n", s)}'

Signed-off-by: Thomas Haller <thaller@redhat.com>
Signed-off-by: Florian Westphal <fw@strlen.de>