Convert gfx to slang-rhi for gfx-unit-test

[gtong-nv]

Currenlty the gfx-unit-test is using gfx, which should be replaced by slang-rhi.
This issue is to track the progress of porting gfx to slang-rhi

• Update tools\CMakeLists.txt to use slang-rhi for the gfx-unit-test, and remove gfx targets
• Remove gfx-util target and src
• Port tools\gfx-test-util.cpp\h and tools\gfx-test-texture-util.cpp\h to use slang-rhi
• Port tests in tools\gfx-unit\test to use slang-rhi