selftests: forwarding: lib: Add require_command()

The logic for testing whether a certain command is available is used
several times in the current code base. The tests in follow-up patches
add more requirements like that.

Therefore extract the logic into a named function, require_command(),
that can be used directly from lib.sh as well as from any test that
wishes to declare dependence on some command.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>