Vulkan: Add tessellation to GetPipelineStage helper The tessellation stages were missing from a helper, which was silently returning an invalid value. Add a test and an assert, which fires before the fix. Test: GFXBench Car Chase Test: GLSLTest_ES31.TessellationTextureBufferAccess Bug: angleproject:7135 Bug: b/218314686 Change-Id: I2bc8d374300fc1470e52affabab7491698c99cee Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3554575 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Charlie Lao <cclao@google.com> Commit-Queue: Cody Northrop <cnorthrop@google.com>