[tests] Add backup ssl sites in case of 429 response. (dotnet#7909)

Our SSL test can fail quite often with a HTTP-429 Too Many Requests
error.  This makes the CI very unstable as we are constantly having
to wait and retry the tests.

Improve this by putting that logic into the test itself.  If we get
an HTTP-429 we should try some other SSL site.

tests/Mono.Android-Tests/System.Net/ProxyTest.cs

@@ -13,7 +13,7 @@ public class ProxyTest {
 		[Test]
 		public void QuoteInvalidQuoteUrlsShouldWork ()
 		{
-			string url      = "http://example.com/?query&foo|bar";
+			string url      = "https://bing.com/?query&foo|bar";
 			var request     = (HttpWebRequest) WebRequest.Create (url);
 			request.Method  = "GET";
 			var response    = (HttpWebResponse) request.GetResponse ();

tests/Mono.Android-Tests/System.Net/SslTest.cs

@@ -83,11 +83,24 @@ public void HttpsShouldWork ()
 		void DoHttpsShouldWork ()
 		{
 			// string url = "https://bugzilla.novell.com/show_bug.cgi?id=634817";
-			string url = "https://encrypted.google.com/";
+			string[] urls = new string[]  {
+				"https://dotnet.microsoft.com/",
+				"https://www.bing.com/",
+				"https://httpbin.org/get",
+			};
 			// string url = "http://slashdot.org";
-			HttpWebRequest request = (HttpWebRequest) WebRequest.Create(url);
-			request.Method = "GET";
-			var response = (HttpWebResponse) request.GetResponse ();
+			HttpWebResponse response = null;
+			foreach (var url in urls) {
+				HttpWebRequest request = (HttpWebRequest) WebRequest.Create(url);
+				request.Method = "GET";
+				response = (HttpWebResponse) request.GetResponse ();
+				if (response.StatusCode == HttpStatusCode.TooManyRequests) {
+					// try the next url.
+					continue;
+				}
+				break;
+			}
+			Assert.IsNotNull (response);
 			int len = 0;
 			using (var _r = new StreamReader (response.GetResponseStream ())) {
 				char[] buf = new char [4096];