[Tests] Try different urls before failing the test. (#6237)

We do have a test that fails when there are issues accessing the remote
resource. Try different urls to ensure that it is not a network issue with a specific domain.

Fixes: xamarin/maccore#1725

tests/monotouch-test/Foundation/NSDataTest.cs

@@ -155,10 +155,27 @@ public void Https ()
 				Assert.Ignore ("NSData.FromUrl doesn't seem to work in watchOS");
 			}
 #endif
-			using (var url = new NSUrl ("https://www.microsoft.com/robots.txt"))
-			using (var x = NSData.FromUrl (url)) {
-				Assert.That ((x != null) && (x.Length > 0));
+			// we have network issues, try several urls, if one works, be happy, else fail
+			var urls = new string [] {
+				"https://www.microsoft.com/robots.txt",
+				"https://www.xamarin.com/robots.txt",
+				"http://www.bing.com/robots.txt",
+				"https://www.xbox.com/robots.txt",
+				"https://www.msn.com/robots.txt",
+				"https://visualstudio.microsoft.com/robots.txt",
+			};
+			for (var i = 0; i < urls.Length; i++) {
+				NSError error;
+				using (var nsUrl = new NSUrl (urls [i]))
+				using (var x = NSData.FromUrl (nsUrl, NSDataReadingOptions.Uncached, out error)) {
+					if (error != null)
+						continue;
+					Assert.That (x != null);
+					Assert.That (x.Length > 0);
+					return;
+				}
 			}
+			Assert.Fail ("None of the urls could be fetch.");
 		}
 
 		[Test]