selftests/bpf: Time tc_edt from its first packet with data

tc_edt sends 1MB over a TCP connection that a BPF program shapes with
EDT, and checks that the rate it measures is within 2% of the target
rate. The transfer takes about 200ms, so the margin is about 4ms.

The measurement starts before send_recv_data(), which first creates the
sender thread and accepts the connection. On a KASAN kernel, x86_64 VM
with 32 vCPUs, that takes 0.7ms when idle and up to 4.7ms with all
CPUs busy, while the BPF program spreads the data over 200.08ms in the
median. The test also fails without test_progs -j where everything runs
slower, e.g. on s390x and with ASAN in BPF CI.

Have the BPF program note when the first packet with data goes out, and
start the measurement there.

tc_edt run 100 times on the same VM, idle and with 32 busy loops in the
background, before and after this patch:

                 before                after
  idle           0 of 100 failed       0 of 100 failed
  busy loops     3 of 100 failed       0 of 100 failed

Fixes: b0f82e7ab6fb ("selftests/bpf: integrate test_tc_edt into test_progs")
Signed-off-by: Qiliang Yuan <odys.yuan@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: Alexis Lothoré (eBPF Foundation) <alexis.lothore@bootlin.com>
Link: https://patch.msgid.link/20261019-selftests-bpf-tc-edt-serial-v2-1-41c3813e7916@gmail.com