[browser] improve HTTP abort test (#118274)

Author: pavelsavara

src/libraries/Common/tests/System/Net/Http/ResponseStreamTest.cs

@@ -319,8 +319,18 @@ public async Task BrowserHttpHandler_StreamingResponseAbort(HttpMethod method, s
             using HttpClient client = CreateHttpClientForRemoteServer(Configuration.Http.RemoteHttp11Server);
             if (abort == "abortDuringBody")
             {
-                using var res = await client.SendAsync(req, HttpCompletionOption.ResponseHeadersRead);
+                HttpResponseMessage res;
+                try
+                {
+                    res = await client.SendAsync(req, HttpCompletionOption.ResponseHeadersRead);
+                }
+                catch (HttpRequestException)
+                {
+                    // sometimes the server aborts earlier than the browser is able to return from the non-blocking send/fetch
+                    return;
+                }
                 await Assert.ThrowsAsync<HttpRequestException>(() => res.Content.ReadAsByteArrayAsync());
+                res.Dispose();
             }
             else
             {

src/libraries/Common/tests/System/Net/Prerequisites/NetCoreServer/Handlers/EchoHandler.cs

@@ -85,7 +85,7 @@ public static async Task InvokeAsync(HttpContext context)
                 if (qs.Contains("abortDuringBody"))
                 {
                     await context.Response.Body.FlushAsync();
-                    await Task.Delay(10);
+                    await Task.Delay(100);
                     context.Abort();
                     return;
                 }