selftests: add a tc matchall test case

This is a follow up of the commit 0db6f8be ("net/sched: fix ->get
helper of the matchall cls").

To test it:
$ cd tools/testing/selftests/tc-testing
$ ln -s ../plugin-lib/nsPlugin.py plugins/20-nsPlugin.py
$ ./tdc.py -n -e 2638

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>