Test shader definitions field in shader reload soak test

[jeremyong-az]

This commit adds a new test shader file FullscreenMacroWhite.shader.txt which adds a preprocessor definition "WHITE" to the shader. The red, blue, and green shader source files all conditionally emit 0xFFFFFFFF if this macro is defined, and the shader reload test checks for a white pixel if this shader is used.

Result after running newly modified shader reload soak test.

Requires https://github.com/o3de/o3de/pull/7356