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_PSVersionInfo.cs

@@ -0,0 +1,17 @@
+using Xunit;
+using System;
+using System.Management.Automation;
+
+namespace PSTests
+{
+    public static class PSVersionInfoTests
+    {
+        [Fact]
+        public static void TestVersions()
+        {
+            // test that a non-null version table is returned, and
+            // that it does not throw
+            Assert.NotNull(PSVersionInfo.GetPSVersionTable());
+        }
+    }
+}