selftests/tc-testing: Fix cannot create /sys/bus/netdevsim/new_device: Directory nonexistent

Install netdevsim to provide /sys/bus/netdevsim/new_device interface.

It helps to fix:
 # ok 97 9a7d - Change ETS strict band without quantum # skipped - skipped - previous setup failed 11 ce7d
 #
 #
 # -----> prepare stage *** Could not execute: "echo "1 1 4" > /sys/bus/netdevsim/new_device"
 #
 # -----> prepare stage *** Error message: "/bin/sh: 1: cannot create /sys/bus/netdevsim/new_device: Directory nonexistent
 # "
 #
 # -----> prepare stage *** Aborting test run.
 #
 #
 # <_io.BufferedReader name=5> *** stdout ***
 #

Signed-off-by: Li Zhijian <zhijianx.li@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>