More test coverage

[walbourn]

Reviewing the code coverage data, here are some additional needed cases:

• GraphicsMemory::GarbageCollect

• EffectsTest

  • BasicEffect::SetColorAndAlpha

• ModelTest

  • CreateFromCMO with ModelLoader_MaterialColorsSRGB

• PBRTest

  • PBREffect::SetAlbedoTexture, SetNormalTexture, SetRMATexture, SetRenderTargetSizeInPixels

• ShaderTest

  • NormalMapEffect::SetColorAndAlpha, SetEmissiveColor

• DDS

  • DepthStencil
  • DXGI_FORMAT_NV11 (not widely supported by hardware or WARP)
  • Skip mips
  • LoadDDSTextureFromMemory / LoadDDSTextureFromMemoryEx
  • LoadDDSTextureFromFile / LoadDDSTextureFromFileEx
  • CreateDDSTextureFromMemoryEx mipgen and alpha mode

• WIC

  • LoadWICTextureFromMemory / LoadWICTextureFromMemoryEx
  • LoadWICTextureFromFile / LoadWICTextureFromFileEx
  • CreateWICTextureFromMemoryEx mipgen

Also need 'nullptr' and invalid arg tests coverage for many functions to get branch coverage up.