Fix test build breaks for netfx.

src/System.Text.Json/tests/Resources/Strings.resx

@@ -20268,4 +20268,7 @@ tiline\"another\" String\\"],"str":"\"\""}</value>
   }
 }</value>
   </data>
+  <data name="BufferWriterAdvancedTooFar" xml:space="preserve">
+    <value>Cannot advance past the end of the buffer, which has a size of {0}.</value>
+  </data>
 </root>

src/System.Text.Json/tests/Utf8JsonWriterTests.cs

@@ -596,7 +596,10 @@ private static async Task WriteLargeToStreamHelper(Stream stream, JsonWriterOpti
         {
             const int SyncWriteThreshold = 25_000;
 
-            await using var jsonUtf8 = new Utf8JsonWriter(stream, options);
+#if !netstandard
+            await
+#endif
+                using var jsonUtf8 = new Utf8JsonWriter(stream, options);
 
             byte[] utf8String = Encoding.UTF8.GetBytes("some string 1234");