libpng: Add ptest for libpng

libpng: Add ptest for libpng

libpng is a platform-independent library which
supports all PNG features.
This ptest executes the below binaries, parses
the png image and prints the image features.

1. pngfix - provides information about PNG image
copyrights details.

2. pngtest - tests, optimizes and optionally fixes
the zlib header in PNG files.

3. pngstest - verifies the integrity of PNG image by
dumping chunk level information.

4. timepng - provides details about PNG image chunks.

Signed-off-by: Nikhil R <nikhil.r@kpit.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>