Update src/tint unittests to new @stage format.

This CL updates all of the Tint unittests to the new @stage shorter
syntax. This also updates the WGSL writer to emit the new short forms
instead of using the deprecated form.

Bug: tint:1503
Change-Id: I8c49e5319a19cccb5b4b5078f3ab39c50f31a9a8
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/92483
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: Dan Sinclair <dsinclair@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>

test/tint/unicode/comments.wgsl

@@ -5,7 +5,7 @@
 नमस्ते दुनिया
 */
 
-@stage(fragment)
+@fragment
 fn /* こんにちは世界 */ main( /* 你好世界 */ ) {
     // 안녕하세요 세계
 }

test/tint/unicode/comments.wgsl.expected.wgsl

@@ -1,3 +1,3 @@
-@stage(fragment)
+@fragment
 fn main() {
 }

test/tint/unicode/identifiers.wgsl

@@ -5,7 +5,7 @@ fn 𝓯𝓾𝓷𝓬𝓽𝓲𝓸𝓷(ᵖᵃʳᵃᵐ : 𝓉𝓎𝓅ℯ_𝒶) ->
     return 𝐭𝐲𝐩𝐞_𝐛(ᵖᵃʳᵃᵐ);
 }
 
-@stage(fragment)
+@fragment
 fn 𝕖𝕟𝕥𝕣𝕪𝕡𝕠𝕚𝕟𝕥() {
     var 𝙫𝙖𝙧𝙞𝙖𝙗𝙡𝙚 : 𝓉𝓎𝓅ℯ_𝒶;
     let 𝖗𝖊𝖘𝖚𝖑𝖙 = 𝓯𝓾𝓷𝓬𝓽𝓲𝓸𝓷(𝙫𝙖𝙧𝙞𝙖𝙗𝙡𝙚);

test/tint/unicode/identifiers.wgsl.expected.wgsl

@@ -6,7 +6,7 @@ fn 𝓯𝓾𝓷𝓬𝓽𝓲𝓸𝓷(ᵖᵃʳᵃᵐ : 𝓉𝓎𝓅ℯ_𝒶) ->
   return 𝐭𝐲𝐩𝐞_𝐛(ᵖᵃʳᵃᵐ);
 }
 
-@stage(fragment)
+@fragment
 fn 𝕖𝕟𝕥𝕣𝕪𝕡𝕠𝕚𝕟𝕥() {
   var 𝙫𝙖𝙧𝙞𝙖𝙗𝙡𝙚 : 𝓉𝓎𝓅ℯ_𝒶;
   let 𝖗𝖊𝖘𝖚𝖑𝖙 = 𝓯𝓾𝓷𝓬𝓽𝓲𝓸𝓷(𝙫𝙖𝙧𝙞𝙖𝙗𝙡𝙚);