[clang-format] Add a test showing the current config file list parsing

Currently clang-format starts overriding the default values at index 0
(keeping the existing values) instead of appending or replacing all values.
This patch simply checks the current (IMO surprising) behaviour and does
not attempt to change it.

Reviewed By: MyDeveloperDay

Differential Revision: https://reviews.llvm.org/D86941