ci: exclude TestPBRIrradianceHDR in wasm

- Add it to the list of PBR tests which are excluded
- It doesn't work because in webgl2, GL_RGB32F is non-colorable and non-filterable. This was identified in vtk/vtk@de4ce1be, and handled properly.
- Although, back then, the regression test framework used a high threshold. This explains why `TestPBRIrradianceHDR` passed even though the `webgl2` code path used integer textures and no mipmaps.