selftests: forwarding: lib: Move require_command to net, generalize

This helper could be useful to more than just forwarding tests.
Move it upstairs and port over to log_test_skip().

Split the function into two parts: the bit that actually checks and
reports skip, which is in a new function check_command(). And a bit
that exits the test script if the check fails. This allows users
consistent checking behavior while giving an option to bail out from
a single test without bailing out of the whole script.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>