selftests: tc: Add basic mpls_* matching support for tc-flower

Add tests in tc_flower.sh for mpls_label, mpls_tc, mpls_bos and
mpls_ttl. For each keyword, test the minimal and maximal values.

Selectively skip these new mpls tests for tc versions that don't
support them.

Signed-off-by: Guillaume Nault <gnault@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>