selftests/bpf: Fix bpf_nf failure upon test rerun

Alexei reported:

  After fast forwarding bpf-next today bpf_nf test started to fail when
  run twice:

  $ ./test_progs -t bpf_nf
  #17      bpf_nf:OK
  Summary: 1/10 PASSED, 0 SKIPPED, 0 FAILED

  $ ./test_progs -t bpf_nf
  All error logs:
  test_bpf_nf_ct:PASS:test_bpf_nf__open_and_load 0 nsec
  test_bpf_nf_ct:PASS:iptables-legacy -t raw -A PREROUTING -j CONNMARK
  --set-mark 42/0 0 nsec
  (network_helpers.c:102: errno: Address already in use) Failed to bind socket
  test_bpf_nf_ct:FAIL:start_server unexpected start_server: actual -1 < expected 0
  #17/1    bpf_nf/xdp-ct:FAIL
  test_bpf_nf_ct:PASS:test_bpf_nf__open_and_load 0 nsec
  test_bpf_nf_ct:PASS:iptables-legacy -t raw -A PREROUTING -j CONNMARK
  --set-mark 42/0 0 nsec
  (network_helpers.c:102: errno: Address already in use) Failed to bind socket
  test_bpf_nf_ct:FAIL:start_server unexpected start_server: actual -1 < expected 0
  #17/2    bpf_nf/tc-bpf-ct:FAIL
  #17      bpf_nf:FAIL
  Summary: 0/8 PASSED, 0 SKIPPED, 1 FAILED

I was able to locally reproduce as well. Rearrange the connection teardown
so that the client closes its connection first so that we don't need to
linger in TCP time-wait.

Fixes: e81fbd4c ("selftests/bpf: Add existing connection bpf_*_ct_lookup() test")
Reported-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/CAADnVQ+0dnDq_v_vH1EfkacbfGnHANaon7zsw10pMb-D9FS0Pw@mail.gmail.com
Link: https://lore.kernel.org/bpf/20230626131942.5100-1-daniel@iogearbox.net