Change async void System.Text.Json test to be async Task (#44418)

src/libraries/System.Text.Json/tests/Serialization/Stream.WriteTests.cs

@@ -404,7 +404,7 @@ public static async Task NestedJsonFileCircularDependencyTest(int depthFactor)
         [InlineData(8192)]
         [InlineData(16384)]
         [InlineData(65536)]
-        public static async void FlushThresholdTest(int bufferSize)
+        public static async Task FlushThresholdTest(int bufferSize)
         {
             // bufferSize * 0.9 is the threshold size from codebase, subtract 2 for [" characters, then create a 
             // string containing (threshold - 2) amount of char 'a' which when written into output buffer produces buffer