Reorganize tests

- Pester source code moved to `test/Pester`, deleted `ext-src`.
- Pester tests (.ps1 files) moved to `test/powershell`
- xUnit tests (.cs files) moved to `test/csharp`
- Third-party script test moved to `test/shebang`

test/csharp/test_ExtensionMethods.cs

@@ -0,0 +1,16 @@
+using Xunit;
+using System;
+using System.Management.Automation;
+
+namespace PSTests
+{
+    public static class PSTypeExtensionsTests
+    {
+        [Fact]
+        public static void TestIsComObject()
+        {
+            // It just needs an arbitrary type
+            Assert.False(PSTypeExtensions.IsComObject(42.GetType()));
+        }
+    }
+}