Add shadow sampler LOD tests for cubemaps Added tests for GLSL 3.00 shadow samplers in fragment shaders with TEXTURE_CUBE_MAP target. Bug: angleproject:365066518 Change-Id: I2b8577310ea5c258d4d13611607f5c4d9be57e04 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/5865112 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>