selftests: add test script for bareudp tunnels

selftests: add test script for bareudp tunnels

Test different encapsulation modes of the bareudp module:
  * Unicast MPLS,
  * IPv4 only,
  * IPv4 in multiproto mode (that is, IPv4 and IPv6),
  * IPv6.

Each mode is tested with both an IPv4 and an IPv6 underlay.

v2:
  * Add build dependencies in config file (Willem de Bruijn).
  * The MPLS test now uses its own IP addresses. This minimises
    the amount of cleanup between tests and simplifies the script.
  * Verify that iproute2 supports bareudp tunnels before running the
    script (and other minor usability improvements).

Signed-off-by: Guillaume Nault <gnault@redhat.com>
Link: https://lore.kernel.org/r/8abc0e58f8a7eeb404f82466505a73110bc43ab8.1604088587.git.gnault@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>