kselftest/runner.sh: add netns support

Add a variable RUN_IN_NETNS if the user wants to run all the selected tests
in namespace in parallel. With this, we can save a lot of testing time.

Note that some tests may not fit to run in namespace, e.g.
net/drop_monitor_tests.sh, as the dwdump needs to be run in init ns.

I also added another parameter -p to make all the logs reported separately
instead of mixing them in the stdout or output.log.

Nit: the NUM in run_one is not used, rename it to test_num.

Acked-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>