[tests] mark hanging test fixture [NonParallelizable] (#4621)

Hanging test prints:

	[TESTLOG] Test CheckClassIsReplacedWithMd5 Outcome=Passed

This started happening since 835c59b, but I don't see how that
commit would cause a hang.

Working build (https://build.azdo.io/3669040) prints this after
`CheckClassIsReplacedWithMd5`:

	=> Xamarin.Android.Build.Tests.ResolveSdksTaskTests.LatestTFV_OldTargetSdkVersion

After adding logging, etc. I couldn't figure out if this was a bug in
NUnit or our tests.  Adding `[NonParallelizable]` for the entire
`ResolveSdksTaskTests` test fixture class seems to solve the hang, so
we should just go with this for now.

Author: jonathanpeppers

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/Tasks/ResolveSdksTaskTests.cs

@@ -13,7 +13,7 @@ namespace Xamarin.Android.Build.Tests {
 
 	[TestFixture]
 	[Category ("Node-2")]
-	[Parallelizable (ParallelScope.Self)]
+	[NonParallelizable] // NOTE: This test was hanging without this
 	public class ResolveSdksTaskTests : BaseTest {
 #pragma warning disable 414