selftests: mptcp: add ms units for tc-netem delay

'delay 1' in tc-netem is confusing, not sure if it's a delay of 1 second or
1 millisecond. This patch explicitly adds millisecond units to make these
commands clearer.

Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>