selftests: mptcp: run userspace pm tests slower

Some userspace pm tests failed are reported by CI:

112 userspace pm add & remove address
      syn                                 [ ok ]
      synack                              [ ok ]
      ack                                 [ ok ]
      add                                 [ ok ]
      echo                                [ ok ]
      mptcp_info subflows=1:1             [ ok ]
      subflows_total 2:2                  [ ok ]
      mptcp_info add_addr_signal=1:1      [ ok ]
      rm                                  [ ok ]
      rmsf                                [ ok ]
      Info: invert
      mptcp_info subflows=0:0             [ ok ]
      subflows_total 1:1                  [fail]
                         got subflows 0:0 expected 1:1
Server ns stats
TcpPassiveOpens                 2                  0.0
TcpInSegs                       118                0.0

This patch fixes them by changing 'speed' to 5 to run the tests much more
slowly.

Fixes: 4369c198 ("selftests: mptcp: test userspace pm out of transfer")
Cc: stable@vger.kernel.org
Reviewed-by: Matthieu Baerts <matttbe@kernel.org>
Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Signed-off-by: Mat Martineau <martineau@kernel.org>
Link: https://lore.kernel.org/r/20231025-send-net-next-20231025-v1-1-db8f25f798eb@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>