tc-testing: gitignore, ignore local tdc config file

Comment in tdc_config.py recommends putting customizations in
tdc_config_local.py file that wasn't included in gitignore. Add the local
config file to gitignore.

Signed-off-by: Vlad Buslov <vladbu@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>