Allow null output file in test mode
Allow null output file in test mode

In test mode, specifying "/dev/null" on Linux/macOS, or "NUL" or "nul"
on Windows, will cause the write of the output file to be omitted. This
can substantially improve test thoughput in cases where only the
PSNR metrics are required.

ARM-software/astc-encoder,ARM-software/astc-encoder,ARM-software/astc-encoder,ARM-software/astc-encoder