selftests: fib_rule_tests: Add flow label selector match tests

Add tests for the new FIB rule flow label selector. Test both good and bad
flows and with both input and output routes.

 # ./fib_rule_tests.sh
 IPv6 FIB rule tests
 [...]
    TEST: rule6 check: flowlabel redirect to table                      [ OK ]
    TEST: rule6 check: flowlabel no redirect to table                   [ OK ]
    TEST: rule6 del by pref: flowlabel redirect to table                [ OK ]
    TEST: rule6 check: iif flowlabel redirect to table                  [ OK ]
    TEST: rule6 check: iif flowlabel no redirect to table               [ OK ]
    TEST: rule6 del by pref: iif flowlabel redirect to table            [ OK ]
    TEST: rule6 check: flowlabel masked redirect to table               [ OK ]
    TEST: rule6 check: flowlabel masked no redirect to table            [ OK ]
    TEST: rule6 del by pref: flowlabel masked redirect to table         [ OK ]
    TEST: rule6 check: iif flowlabel masked redirect to table           [ OK ]
    TEST: rule6 check: iif flowlabel masked no redirect to table        [ OK ]
    TEST: rule6 del by pref: iif flowlabel masked redirect to table     [ OK ]
 [...]

 Tests passed: 268
 Tests failed:   0

Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Guillaume Nault <gnault@redhat.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>