Add all classes to AssemblyLoadContext collection

To ensure that the initializer is called before any other code. This is
as close to a global init hook as I can find.

test/csharp/test_ExtensionMethods.cs

@@ -4,6 +4,7 @@ using System.Management.Automation;
 
 namespace PSTests
 {
+    [Collection("AssemblyLoadContext")]
     public static class PSTypeExtensionsTests
     {
         [Fact]