Use .runner_config instead of .config

CTS TF started to read subdir .config file converted from AT.xml in the new suite impl.
This conflicts with our runner config's file extension.

Test: run vts-selftest
Change-Id: Ia04ae442b33094d3cc398f371e071d836195a917
Merged-In: Ia04ae442b33094d3cc398f371e071d836195a917
8 files changed