tests: logging/dictionary: fix pytest when built with sysbuild

With sysbuild, the test app is actually built under another
layer of directory where the dictionary JSON file will reside.
Instead of simply using dut.device_config.build_dir, use
dut.devive_config.app_build_dir instead, which takes into
account of that.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>