selftests/bpf: Skip tc_tunnel subtest if its setup fails

selftests/bpf: Skip tc_tunnel subtest if its setup fails

A subtest setup can fail in a wide variety of ways, so make sure not to
run it if an issue occurs during its setup. The return value is
already representing whether the setup succeeds or fails, it is just
about wiring it.

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/20251031-tc_tunnel_improv-v1-1-0ffe44d27eda@bootlin.com