[Xamarin.Android.Build.Tests] Flush our test output. (dotnet#1714)

Our wrench build if timing out because its not getting any console
output from Nunit.  This is because Nunit is buffering the output.

Flush the Output so we get decent progress messages.

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

@@ -303,12 +303,14 @@ public void FixtureSetup ()
 		public void TestSetup ()
 		{
 			TestContext.Out.WriteLine ($"[TESTLOG] Test {TestName} Starting");
+			TestContext.Out.Flush ();
 		}
 
 		[TearDown]
 		protected virtual void CleanupTest ()
 		{
 			TestContext.Out.WriteLine ($"[TESTLOG] Test {TestName} Complete");
+			TestContext.Out.Flush ();
 			if (System.Diagnostics.Debugger.IsAttached || TestContext.CurrentContext.Test.Properties ["Output"] == null)
 					return;
 			// find the "root" directory just below "temp" and clean from there because