Add tint_unittest flags for enabling DXC validation

HLSL validation will be disabled by default, and is now opt-in with
--validate-hlsl and/or with a --dxc-path

Change-Id: Ia98d7b1fbba50168bbe85c7982b73598100016d7
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/42024
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: dan sinclair <dsinclair@chromium.org>

src/utils/command.h

@@ -52,6 +52,9 @@ class Command {
   /// constructor
   bool Found() const;
 
+  /// @returns the path of the command
+  const std::string& Path() const { return path_; }
+
   /// Invokes the command with the given argument strings, blocking until the
   /// process has returned.
   /// @param args the string arguments to pass to the process