[Xamarin.Android.Build.Tests] Improve logcat logging in failed tests (#7816)

I noticed that more often than not, we don't see the actual error in
`logcat-failed.log` when the test fails on CI, but when it runs
locally, we can see the issue logged properly.

I'm not 100% what causes it on CI, but I suspect that we might be
facing a problem with the logcat buffer wrapping around and
overwriting the interesting data.  This might be because of the fact
that `adb logcat -c` runs only once per *class* containing tests and
also because we log too quickly after the app quits.

To remedy the former, run `adb logcat -c` before each *test* starts.
To attempt a fix for the latter, pause for 1 second before dumping
logcat buffer after the test fails.

Author: grendello

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/Utilities/DeviceTest.cs

@@ -48,7 +48,7 @@ public void AssertHasDevices (bool fail = true)
 		public void DeviceSetup ()
 		{
 			SetAdbLogcatBufferSize (64);
-			RunAdbCommand ("logcat -c");
+			ClearAdbLogcat ();
 			CreateGuestUser (GuestUserName);
 		}
 
@@ -69,6 +69,9 @@ public void CheckDevice ()
 				TestContext.Out.WriteLine ($"{nameof(CheckDevice)} is restarting the emulator.");
 				RestartDevice (Path.Combine (Root, "Emulator.csproj"));
 			}
+
+			// We want to start with a clean logcat buffer for each test
+			ClearAdbLogcat ();
 		}
 
 		[TearDown]
@@ -82,6 +85,9 @@ protected override void CleanupTest ()
 				string remote = "/data/local/tmp/screenshot.png";
 				string localUi = Path.Combine (outputDir, "ui.xml");
 				RunAdbCommand ($"shell screencap {remote}");
+
+				// Give the app a chance to finish logging
+				WaitFor (1000);
 				var output = RunAdbCommand ($"logcat -d");
 				File.WriteAllText (deviceLog, output);
 				RunAdbCommand ($"pull {remote} \"{local}\"");