selftests/tc-testing: add missing config

qdiscs/fq_pie requires CONFIG_NET_SCH_FQ_PIE, otherwise tc will fail
to create a fq_pie qdisc.

It fixes following issue:
 # not ok 57 83be - Create FQ-PIE with invalid number of flows
 #       Command exited with 2, expected 0
 # Error: Specified qdisc not found.

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