Adding validation logic for RGBA16 readpixels.
Change readPixels format from UNSIGNED_BYTE to UNSIGNED_SHORT in the
test.

FYI:
According to https://www.khronos.org/registry/OpenGL/extensions/EXT/EXT_texture_norm16.txt
ReadPixels format and type used during CopyTex* are limited to RGBA

Bug: 1024387, 1000354, angleproject:1365, angleproject:4089
Change-Id: I70517f255fe335f60e55bdf15f7ebf82e3de0800
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1914127Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Shrek Shao <shrekshao@google.com>