[tests] ignore network errors in CustomLinkDescriptionPreserve (#9915)

Context: 455ec0a
Context: https://devdiv.visualstudio.com/DevDiv/_build/results?buildId=11188869&view=ms.vss-test-web.build-test-results-tab&runId=124545813&resultId=100000&paneView=debug

The `InstallAndRunTests.CustomLinkDescriptionPreserve()` test can
randomly fail with:

	I XALINKERTESTS: [FAIL] HttpClientTest.Post FAILED: System.Net.Http.HttpRequestException: net_http_message_not_success_statuscode_reason, 503, Service Temporarily Unavailable
	I XALINKERTESTS:    at System.Net.Http.HttpResponseMessage.EnsureSuccessStatusCode()
	I XALINKERTESTS:    at HttpClientTest.Post()

I updated `HttpClientTest.Post()` to print an `[IGNORE]` result when
it encounters HTTP-500 errors (.InternalServerError, .BadGateway,
.ServiceUnavailable, .GatewayTimeout), similar to 455ec0a.

Author: jonathanpeppers

tests/MSBuildDeviceIntegration/Resources/LinkDescTest/HttpClientTest.cs

@@ -1,23 +1,28 @@
 using System;
 using System.Text;
+using System.Net;
 using System.Net.Http;
 
 public class HttpClientTest
 {
 	// [Test]
 	public static string Post ()
 	{
-		try
-		{
+		try {
 			var client = new HttpClient ();
 			var data = new StringContent ("{\"foo\": \"bar\" }", Encoding.UTF8, "application/json");
 			var response = client.PostAsync ("https://httpbin.org/post", data).Result;
+			switch (response.StatusCode) {
+ 				case HttpStatusCode.InternalServerError:
+ 				case HttpStatusCode.BadGateway:
+ 				case HttpStatusCode.ServiceUnavailable:
+ 				case HttpStatusCode.GatewayTimeout:
+ 					return $"[IGNORE] {nameof (HttpClientTest)}.{nameof (Post)} {response.StatusCode}";
+ 			}
 			response.EnsureSuccessStatusCode ();
 			var json = response.Content.ReadAsStringAsync ().Result;
 			return $"[PASS] {nameof (HttpClientTest)}.{nameof (Post)}";
-		}
-		catch (Exception ex)
-		{
+		} catch (Exception ex) {
 			return $"[FAIL] {nameof (HttpClientTest)}.{nameof (Post)} FAILED: {ex}";
 		}
 	}