tests: use vpx_codec_dec_cfg_t() to initialize vars

tests: use vpx_codec_dec_cfg_t() to initialize vars

0-initializes [1] and removes a warning for missing initializer fields

[1] http://en.cppreference.com/w/cpp/language/value_initialization

Change-Id: I364248010c8fa663c71d8f06a9999c730e92db4c