make Http2_RequestFailsWithAppropriateHttpProtocolException test more…

… deterministic (#94112)

* make Http2_IncorrectServerPreface_RequestFailsWithAppropriateHttpProtocolException test more deterministic

* feedback

src/libraries/System.Net.Http/tests/FunctionalTests/HttpClientHandlerTest.Http2.cs

@@ -139,7 +139,8 @@ public async Task Http2_StreamResetByServerBeforePrefix_RequestFailsWithGoawayPr
 
                 Http2LoopbackConnection connection = await server.AcceptConnectionAsync(timeout: null);
                 _ = await connection.ReadSettingsAsync();
-
+                // Wait until client starts sending request
+                _ = await connection.ReadFrameAsync(TestHelper.PassingTestTimeout);
                 GoAwayFrame goAwayFrame = new GoAwayFrame(lastStreamId: 0, (int)ProtocolErrors.HTTP_1_1_REQUIRED, additionalDebugData: Array.Empty<byte>(), streamId: 0);
                 await connection.WriteFrameAsync(goAwayFrame);
 
@@ -319,6 +320,8 @@ public async Task Http2_IncorrectServerPreface_RequestFailsWithAppropriateHttpPr
 
                 Http2LoopbackConnection connection = await server.AcceptConnectionAsync();
                 await connection.ReadSettingsAsync();
+                // Wait until client starts sending request
+                _ = await connection.ReadFrameAsync(TestHelper.PassingTestTimeout);
                 await connection.SendGoAway(0, ProtocolErrors.INTERNAL_ERROR);
 
                 await AssertProtocolErrorAsync(sendTask, ProtocolErrors.INTERNAL_ERROR);