main: Replace --dawn-validation with --validate

Performs output validation with spirv-val for SPIR-V (as before), HLSL
validation with DXC, and MSL validation with the Metal Shader Compiler.

Disable HLSL tests that fail to validate

Bug: tint:812
Change-Id: If78c351b4e23c7fb50d333eacf9ee7cc81d18564
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/51280
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Antonio Maiorano <amaiorano@google.com>

tools/src/fileutils/fileutils_windows.go

@@ -0,0 +1,21 @@
+// Copyright 2021 The Tint Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package fileutils contains utility functions for files
+package fileutils
+
+// IsExe returns true if the file at path is an executable
+func IsExe(path string) bool {
+	return true
+}