Vulkan: Enable a few polygon offset tests in dEQP These tests pass on Vulkan backend except on Nvidia cards. Nivida vulkan driver does not clamp the depth fragment in case of floating-point representation. Bug: angleproject:3678 Test: dEQP-GLES3.functional.polygon_offset.float32_result_depth_clamp dEQP-GLES3.functional.polygon_offset.float32_factor_1_slope Change-Id: I2be70f98aafac546af592aa55c5b56e836e53a46 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1824797 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>