selftests/bpf: Use start_server_str rather than start_reuseport_server in tc_tunnel

selftests/bpf: Use start_server_str rather than start_reuseport_server in tc_tunnel

Now that start_server_str enforces SO_REUSEADDR, there's no need to keep
using start_reusport_server in tc_tunnel, especially since it only uses
one server at a time.

Replace start_reuseport_server with start_server_str in tc_tunnel test.

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/20251105-start-server-soreuseaddr-v1-2-1bbd9c1f8d65@bootlin.com