tools/selftests: Use a sensible timeout value for iperf3 client

tools/selftests: Use a sensible timeout value for iperf3 client

The default timeout of cmd() is 5 seconds and Iperf3Runner requests the
iperf3 client to run for 10 seconds, which clearly doesn't work since
commit [1] enforced the timeout parameter.

Use a value derived from duration as timeout (+5 seconds for
startup/teardown/various other overhead).

[1] commit f0bd19316663 ("selftests: net: fix timeout passed as positional argument to communicate()")
Signed-off-by: Cosmin Ratiu <cratiu@nvidia.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>