More readPixels validation needed in EXT_texture_norm16 test

[A recent update in the CTS](https://github.com/KhronosGroup/WebGL/pull/3158) exposes some validation error in Chrome implementation. This reminds us there are some readPixels validation test we need to add in the test for EXT_texture_norm16 extension:
* If the bound framebuffer's color attachment is 16 bits per channel, then per https://www.khronos.org/registry/OpenGL/extensions/EXT/EXT_texture_norm16.txt , RGBA / UNSIGNED_SHORT readbacks are allowed. They're not allowed when the framebuffer has an 8-bit-per-channel color attachment. Need to make sure the conformance tests enforce this.
* The IMPLEMENTATION_COLOR_READ_FORMAT and IMPLEMENTATION_COLOR_READ_TYPE might need to change when a 16-bit-per-channel texture is the FBO's color attachment. Need to make sure the conformance tests enforce this.