Http protocol dribble tests taking two long (#26680)

The time of inner loop is dominated by the http protocol dribble tests.

Returning the Task.Delay to Task.Yield reduces it from ~19 seconds to less than a second on a Windows 10 RS3 box.

Author: Paulo Janotti

src/System.Net.Http/tests/FunctionalTests/HttpProtocolTests.cs

@@ -167,7 +167,7 @@ public override async Task WriteAsync(byte[] buffer, int offset, int count, Canc
                 for (int i = 0; i < count; i++)
                 {
                     await _wrapped.WriteAsync(buffer, offset + i, 1);
-                    await Task.Delay(3); // introduce short delays, enough to send packets individually but not so long as to extend test duration significantly
+                    await Task.Yield(); // introduce short delays, enough to send packets individually but not so long as to extend test duration significantly
                 }
             }