selftests/tc-testing: add exit code

Mark the summary result as FAIL to prevent from confusing the selftest
framework if some of them are failed.

Previously, the selftest framework always treats it as *ok* even though
some of them are failed actually. That's because the script tdc.sh always
return 0.

 # All test results:
 #
 # 1..97
 # ok 1 83be - Create FQ-PIE with invalid number of flows
 # ok 2 8b6e - Create RED with no flags
[...snip]
 # ok 6 5f15 - Create RED with flags ECN, harddrop
 # ok 7 53e8 - Create RED with flags ECN, nodrop
 # ok 8 d091 - Fail to create RED with only nodrop flag
 # ok 9 af8e - Create RED with flags ECN, nodrop, harddrop
 # not ok 10 ce7d - Add mq Qdisc to multi-queue device (4 queues)
 #       Could not match regex pattern. Verify command output:
 # qdisc mq 1: root
 # qdisc fq_codel 0: parent 1:4 limit 10240p flows 1024 quantum 1514 target 5ms interval 100ms memory_limit 32Mb ecn drop_batch 64
 # qdisc fq_codel 0: parent 1:3 limit 10240p flows 1024 quantum 1514 target 5ms interval 100ms memory_limit 32Mb ecn drop_batch 64
[...snip]
 # ok 96 6979 - Change quantum of a strict ETS band
 # ok 97 9a7d - Change ETS strict band without quantum
 #
 #
 #
 #
 ok 1 selftests: tc-testing: tdc.sh <<< summary result

CC: Philip Li <philip.li@intel.com>
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Li Zhijian <zhijianx.li@intel.com>
Acked-by: Davide Caratti <dcaratti@redhat.com>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>