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_CorePsExtensions.cs

@@ -0,0 +1,19 @@
+using Xunit;
+using System;
+using System.Management.Automation;
+
+namespace PSTests
+{
+    public static class CorePsExtensionsTests
+    {
+        [Fact]
+        public static void TestEnvironmentOSVersion()
+        {
+            // this test primarily checks that there is no exception thrown
+            // NOTE: the type name is explicitly written here to make clear what
+            //       type and function is tested
+            System.Management.Automation.Environment.OperatingSystem os = System.Management.Automation.Environment.OSVersion;
+            Assert.NotNull(os);
+        }
+    }
+}