selftests/bpf: Integrate test_tc_tunnel.sh tests into test_progs

selftests/bpf: Integrate test_tc_tunnel.sh tests into test_progs

The test_tc_tunnel.sh script checks that a large variety of tunneling
mechanisms handled by the kernel can be handled as well by eBPF
programs. While this test shares similarities with test_tunnel.c (which
is already integrated in test_progs), those are testing slightly
different things:
- test_tunnel.c creates a tunnel interface, and then get and set tunnel
  keys in packet metadata, from BPF programs.
- test_tc_tunnels.sh manually parses/crafts packets content

Bring the tests covered by test_tc_tunnel.sh into the test_progs
framework, by creating a dedicated test_tc_tunnel.sh. This new test
defines a "generic" runner which, for each test configuration:
- will configure the relevant veth pair, each of those isolated in a
  dedicated namespace
- will check that traffic will fail if there is only an encapsulating
  program attached to one veth egress
- will check that traffic succeed if we enable some decapsulation module
  on kernel side
- will check that traffic still succeeds if we replace the kernel
  decapsulation with some eBPF ingress decapsulation.

Example of the new test execution:

  # ./test_progs -a tc_tunnel
  #447/1   tc_tunnel/ipip_none:OK
  #447/2   tc_tunnel/ipip6_none:OK
  #447/3   tc_tunnel/ip6tnl_none:OK
  #447/4   tc_tunnel/sit_none:OK
  #447/5   tc_tunnel/vxlan_eth:OK
  #447/6   tc_tunnel/ip6vxlan_eth:OK
  #447/7   tc_tunnel/gre_none:OK
  #447/8   tc_tunnel/gre_eth:OK
  #447/9   tc_tunnel/gre_mpls:OK
  #447/10  tc_tunnel/ip6gre_none:OK
  #447/11  tc_tunnel/ip6gre_eth:OK
  #447/12  tc_tunnel/ip6gre_mpls:OK
  #447/13  tc_tunnel/udp_none:OK
  #447/14  tc_tunnel/udp_eth:OK
  #447/15  tc_tunnel/udp_mpls:OK
  #447/16  tc_tunnel/ip6udp_none:OK
  #447/17  tc_tunnel/ip6udp_eth:OK
  #447/18  tc_tunnel/ip6udp_mpls:OK
  #447     tc_tunnel:OK
  Summary: 1/18 PASSED, 0 SKIPPED, 0 FAILED

Signed-off-by: Alexis Lothoré (eBPF Foundation) <alexis.lothore@bootlin.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://patch.msgid.link/20251027-tc_tunnel-v3-3-505c12019f9d@bootlin.com