Fix HttpClientHandler_ServerCertificates_Test (#49130)

Author: antonfirsov

src/libraries/Common/tests/System/Net/Http/HttpClientHandlerTest.ServerCertificates.cs

@@ -33,6 +33,7 @@ public HttpClientHandler_ServerCertificates_Test(ITestOutputHelper output) : bas
         // This enables customizing ServerCertificateCustomValidationCallback in WinHttpHandler variants:
         protected bool AllowAllHttp2Certificates { get; set; } = true;
         protected new HttpClientHandler CreateHttpClientHandler() => CreateHttpClientHandler(UseVersion, allowAllHttp2Certificates: AllowAllHttp2Certificates);
+        protected override HttpClient CreateHttpClient() => CreateHttpClient(CreateHttpClientHandler());
 
         [Fact]
         public void Ctor_ExpectedDefaultValues()