selftests: forwarding: Only check tc version for tc tests

Capabilities of tc command are irrelevant for router tests:
    $ ./router.sh
    SKIP: iproute2 too old, missing shared block support

Add a CHECK_TC flag and only check tc capabilities if set. Add flag to
tc_common.sh and have it sourced before lib.sh

Also, if the command lacks some feature the test should exit non-0.

Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>