[tests] Fix recently added AndroidMessageHandler test (#7859)

Context: 5d46685050
Context: 7b2e172829

Commit 5d466850 -- via [7b2e17][0] -- added usage of the NUnit3
[`RetryAttribute`][1] to some of our on-device unit tests.
Unfortunately, `RetryAttribute` doesn't exist in NUnitLite, resulting
in build failures such as:

	…/tests/Mono.Android-Tests/Xamarin.Android.Net/AndroidMessageHandlerTests.cs(42,4):
	error CS0246: The type or namespace name 'RetryAttribute' could not be found (are you missing a using directive or an assembly reference?)

Oops.

This failure wasn't caught because we've trained ourselves to
partially ignore various failures in the **Tests** tab  -- networking
is hard, mkay? -- but completely missed the *compilation* failures,
which don't appear in the **Tests** tab and are harder to see.

Double oops.

Update `AndroidMessageHandlerTests.cs` to remove usage of `[Retry]`
and instead retry things "manually"

[0]: 7b2e172829
[1]: https://docs.nunit.org/articles/nunit/writing-tests/attributes/retry.html

tests/Mono.Android-Tests/Xamarin.Android.Net/AndroidMessageHandlerTests.cs

@@ -39,20 +39,29 @@ namespace Xamarin.Android.NetTests
 #if NET
 		[Test]
 		[TestCaseSource (nameof (DecompressionSource))]
-		[Retry (5)]
+		// Disabled because it doesn't exist in NUnitLite, uncomment when/if we switch to full NUnit
+		// When we can use it, replace all the Console.WriteLine calls with Assert.Warn
+		// [Retry (5)]
 		public async Task Decompression (string urlPath, string encoding, string jsonFieldName)
 		{
 			// Catch all the exceptions and warn about them or otherwise [Retry] above won't work
 			try {
-				DoDecompression (urlPath, encoding, jsonFieldName);
+				int count = 0;
+				// Remove the loop when [Retry] can be used
+				while (count < 5) {
+					if (await DoDecompression (urlPath, encoding, jsonFieldName)) {
+						return;
+					}
+					count++;
+				}
 			} catch (Exception ex) {
-				Assert.Warn ("Unexpected exception thrown");
-				Assert.Warn (ex.ToString ());
+				Console.WriteLine ("Unexpected exception thrown");
+				Console.WriteLine (ex.ToString ());
 				Assert.Fail ("Exception should have not been thrown");
 			}
 		}
 
-		void DoDecompression (string urlPath, string encoding, string jsonFieldName)
+		async Task<bool> DoDecompression (string urlPath, string encoding, string jsonFieldName)
 		{
 			var handler = new AndroidMessageHandler {
 				AutomaticDecompression = DecompressionMethods.All
@@ -66,7 +75,9 @@ namespace Xamarin.Android.NetTests
 			// we will sleep a short while before failing the test
 			if (!response.IsSuccessStatusCode) {
 				System.Threading.Thread.Sleep (1000);
-				Assert.Fail ($"Request ended with a failure error code: {response.StatusCode}");
+				// Uncomment when we can use [Retry]
+				//Assert.Fail ($"Request ended with a failure error code: {response.StatusCode}");
+				return false;
 			}
 
 			foreach (string enc in response.Content.Headers.ContentEncoding) {
@@ -77,13 +88,15 @@ namespace Xamarin.Android.NetTests
 
 			string responseBody = await response.Content.ReadAsStringAsync ();
 
-			Assert.Warn ("-- Retrieved JSON start");
-			Assert.Warn (responseBody);
-			Assert.Warn ("-- Retrieved JSON end");
+			Console.WriteLine ("-- Retrieved JSON start");
+			Console.WriteLine (responseBody);
+			Console.WriteLine ("-- Retrieved JSON end");
 
 			Assert.IsTrue (responseBody.Length > 0, "Response was empty");
 			Assert.AreEqual (response.Content.Headers.ContentLength, responseBody.Length, "Retrieved data length is different than the one specified in the Content-Length header");
 			Assert.IsTrue (responseBody.Contains ($"\"{jsonFieldName}\"", StringComparison.OrdinalIgnoreCase), $"\"{jsonFieldName}\" should have been in the response JSON");
+
+			return true;
 		}
 #endif